How do I speak with my professor about my mental health? I'm an undergraduate in my junior year, and I've been having an especially bad month when it comes to my mental health and academic performance. Two weeks ago, I discovered that I had two midterms, one major project, and an 6-8 page essay due on the same day. I explained my workload to the professor who assigned the essay, and he was kind enough to give me an indefinite extension on the essay (as in, I could "turn it in whenever it was ready", although I assumed that 1-2 weeks was what he would prefer). Since I was granted that extension, I've completely flunked the major project and received C's on the two midterms, all of which I prepared extensively for once I didn't have to worry about the essay. The professor has started hinting that he'd like essays he gave extensions on (he gave extensions to about half the class, since a good 2/3 of our university got the flu around then) to be turned in soon, since he's expecting to receive more essays from another class he's teaching, and doesn't want to be overwhelmed by grading. 

It's been two weeks since he gave me the extension. I've spent nearly all of those two weeks attempting to write the essay and then completely breaking down. I start thinking about how kind he was to give me an extension in the first place, and how, by not turning it in by now I'm taking advantage of his kindness, and how my reputation among my department's professors will be ruined. I think about how this essay has to be perfect, since I've completely failed my assignments for every other class, and I have to get this one thing right. I wrote 4 pages of the essay and then deleted them all. I wrote 3 pages of the same paragraph with slightly different wording each time in an attempt to get it perfect. My outline (for this 6-8 page paper!) is 7 pages long. But I have no paper, and I have other major midterms coming up that really just compound my panic over this.

So my question is: how do I talk about this issue with the professor? I want him to know that I'm not trying to take advantage of him or slack off, and that I do actually know the material (as my ridiculously detailed outline shows), but I feel like walking into his office hours and telling him about how my nightly panic attacks are keeping me from writing this essay is...inappropriate? Should I mention my mental health issues at all, or will that seem like some sort of weak excuse?
A number of the things you mentioned you've been thinking about the situation, i.e. you're taking advantage of the teacher's kindness, how your reputation will be ruined etc possibly fall under the category of unhelpful thinking styles or cognitive distortions which are common misguided ways in which people think that often leads to an unhelpful outcome. People with depression and anxiety can be particularly bad at thinking in these misguided ways. 

I would recommend seeing a professional mental health expert like a GP or Psychologist as soon as possible and not waiting one month for the councillor. There are lots of tools that professionals can teach to help manage stressful situations like these. Seeing a professional doesn't imply that there is anything wrong with you either. From your post, it seems like you're having a natural reaction to an incredibly high workload and that you haven't gone completely 'off the rails' so to speak. Seeing a professional can help prevent things from getting any worse.

In the meantime, I think the action you should take becomes clearer if you acknowledge that your previous thoughts might be the result of unhelpful thinking patterns. For example: You don't actually know that your reputation is going to be ruined. That impression is most likely the result of some assumptions about someone's reactions. In truth, there are a number of possible outcomes and it is futile to worry about the one possible negative outcome as if that outcome is certain.

I think that the best thing for you to do is to sit down and write out everything you need to do, how long roughly it will take to do and when you are going to do it. If you really need an extension, go and ask the teacher for one and explain the situation and the action you are taking to fix it. 

You might find worksheets such as these useful to reduce anxiety levels about the perceived consequences of actions.