How to cut ties with abusive supervisor? While I read through a similar post written by K.Grayson, I couldn't find a response that overlaps with my case. 

In all honesty, I don't know in what way to explain how the situation with my M.Sc. graduate supervisor has escalated to what it has gotten. There are a lot of factors and interference by people who have nothing to do with our problems that muddled communication. Things escalated to a point where I was on the receiving end of a racist declaration from him, at which point a few breaches of my privacy had set me up on a vulnerable path.

My graduate supervisor has actually admitted to some of the abuse and said that he wouldn't do it again with his new students. (At this point, I suspect that someone in the department had found out, ousted his behavior, leading to some disciplinary action.) Unfortunately, by then, my time was up and I had already submitted my thesis, which I later defended successfully.

Leaving the lab, I actually gained nothing more than whom not to work with and whom not to be in the future. As far as my scientific development goes, I gained absolutely nothing, as extreme as that sounds. I figured since my graduate supervisor had at least admitted to some of his mistakes (mainly withholding information and exclusion from project development discussions, grant/article writing, and experiment designing) that he wouldn't stop me from getting that training I had missed somewhere else. I was wrong. In applying to a PhD program in my field, I had asked him if he could write a strong recommendation later (through e-mail) for me, explaining that this is a field I would like to dedicate myself to, and asked if he could be objective despite his personal reservations. He wrote that he would. 

I was rejected, unfortunately. I grew suspicious, however, of the rejection. I went back to the previously mentioned e-mail, where he had forwarded the PhD department's request for the letter. (He forwarded the e-mail to me initially to convince me that the deadline was not what I was informing him to be. I sent him a link to the department's website to see for himself that I wasn't lying to him.) In the e-mail there was a link, and I clicked on it and read the letter. I know that what I did was unethical, and there is probably no excuse that I can offer to justify it. I know why I did it and I have come to terms with that. However, now I also know that my graduate supervisor does and will not have my best interest at heart, and thus I cannot trust him. I also know that, given that most of the things that have happened with him would require that witnesses corroborate my experience, I essentially cannot relate what I went through in that lab.

I am currently seeking psychiatric help since the culmination of what happened at that lab has left me cynical, enraged, and depressed. I am also having a hard time with the psychiatrist who suspects that I have gone through a psychotic break that left me delusional. Honestly, I don't want retribution or anything; I just want to move on and continue research in my field of interest. It feels like all doors are closing on me, and I just want a clean break from anything that has to do with him and his influence. Is it even worth staying in the same field? If even a psychiatrist is skeptical that a graduate supervisor would do that, is this uphill battle too steep for me to climb?

Thanks.

Edit:  I provided the background info I thought necessary for a stranger to understand my position right now. To clarify my question: what are my chances of getting into and continuing research in the same field considering that:
1. I don't have the primary reference expected by grad schools to be the best source of information on my performance; and
2. I cannot relay what has happened since I am afraid it would be dismissed the same way as with my psychiatrist?

Also, Dawn brings up a good question. How do I go about building new research mentors using a diplomatic approach to discuss my M.Sc. experience?
I was going to just leave this as a comment, but I feel like it's worth an answer (even if it's effectively only one half of the answer), given the other current one:

Your current therapist (psychiatrist) has tried to guide you through this in a way that has caused you to mistrust them, by calling your account into doubt in a way that questions your grounding in reality and as a sole means of directing that line of inquiry, rather than something such as a strategy leading you to find (should they even exist) other symptoms that can serve as evidence of differences between what you recall and what may have actually occurred, should there even be any such discrepancies. All they have done is put you purely on the defensive and add an element of antagonism to a situation where you are already highly pressured from external sources, and where your issues stem from a situation where your trust was horribly abused. From how you are discussing the experience, it sounds like you are unlikely to receive meaningful therapy with them at this point, if trust with your therapist can't be re-established.

Find a good Clinical Psychologist. (PsyD or PhD)

If you do not have other, local avenues of seeking a referral you trust (and assuming that this is in the US), the APA maintains a Psychologist Locator service. You should probably first check if there aren't any state based referral networks available to you, if you can't get a personal referral from someone you trust. One avenue can be through your University's psychology department, not necessarily in terms of seeking care but at least a trusted referral.

A good psychologist will also be positioned to help you move on socially and professionally/academically from this as specific to your situation, with setting up both coping strategies and boundaries, and helping reflect on your own behavior, thoughts, and feelings in relation to both what happened and in moving forward from it. Not every psychologist is a perfect fit for everyone, and not every form of therapy as administered is a perfect fit personally or situationally: it's ok to tell a psychologist who you're trying to work with that things aren't working out with them and you feel you need to see someone else. Every clinical psychologist I personally know would be more than happy to refer you to a colleague who might be a better fit, in that situation.

If you feel you need medical (psychopharmaceutical) support to help manage things like immediately severe (acute) emotional issues while working on recovering, then seek a referral to a psychiatrist for that. There's nothing wrong with needing more or more immediate help than what therapy alone can offer, but I would suggest being upfront with your psychologist about that too (and usually they can be a good source of a referral to someone who can handle the medical side of things in a balanced manner) as it will affect how therapy proceeds in relation to corresponding changes.

A Psychologist will also be able to administer any well validated instruments that are appropriate, or refer you to someone who specializes in related testing, if there is a need for any. 


  I cannot relay what has happened since I am afraid it would be
  dismissed the same way as with my psychiatrist?


This is, ultimately, part of why this answer focuses on seeking out a psychologist to help you through this, because while your focus is on what happened (which is completely understandable), you may need to work on processing it as something unfair and unjust that happened to you and work on moving past it without relying on the particulars of the event as justification for how you choose to do so.

Sometimes when we are in the middle of a situation, all of our attention remains riveted on what happened, and we lose sight of simple answers to moving forward where what happened isn't a relevant matter to doing so. Some of @aparente001's answer focuses on suggestions that would fall under this.

I can think of at least a couple of Psychology Professors who also do clinical work, who I'm personally well acquainted with who have "seen it all" in academia and would give whatever story you have complete face value credence: if you can't find someone willing to give what happened to you a fair listening without immediately jumping to questioning your grounding in reality simply on a basis of what happened, then keep looking, because they are out there. At the same time, it's important to understand that questions of whether your perception of what happened might differ from the actual events is also not necessarily an attack on your state of mind. We are all prone to remembering things in ways that are not perfectly genuine to actual events. 

Someone who approaches that line of questioning should manage to do so in a way that doesn't leave you completely distrusting them, however.


  Things escalated to a point where I was on the receiving end of a racist declaration from him


It's unfortunate that you apparently have no documentation of your supervisor's behavior (although there may be more evidence of that behavior than you believe exists), because I realize the reality is that it can make filing a grievance difficult. I do think it's likely that you probably should file one either way (not about the letter, but about the behavior you experienced) from even the little you've relayed here, depending on the political situation within the department, but given how that may impact you professionally it's probably best to explore a discussion of that with someone you trust and with whom you can discuss it confidentially (which again, leads to the recommendation of seeking out a clinical psychologist you can form a strong working relationship with). I think if I had friends/allies in the department who were familiar with the Dean in particular and the political situation with your adviser in relation to the rest of the department, I would sound things out with them after receiving some counseling, and then make a decision of how to proceed with anything more formal based on their feedback.