Throwaway because, well... you know.

TL;DR Below

I typed this out a couple of times because I felt the context was really important, but it gets way too big... so feel free to INFO if you feel like this isn't detailed enough.

Me(T)22 and my Ex(D)20 were together for about 4 years.

Before D and I got together she was in a very emotionally, and physically abusive, and sexually coercive relationship with a Guy we'll call M. It took about a year's worth of love and therapy, at the end of which we got together, to get her out of the deep depression she was in after he dumped her.

When started Dating, M suddenly took interest in D again, he continuously pestered her to be friends with him (which I was adamantly against. Might sound controlling of me, but after learning the kinds of things he did to D and other girls I just couldn't allow him to be near her). After 2 years of this D agreed to be friends with him - D's therapist WHO KNEW THE THINGS HE DID advised that D befriend him as a means of "letting go" of the past and forgiving him.

I voiced my objections to this the moment I found out, but eventually accepted it because it was what D felt she wanted. 

Over the course of the next 2 years, D would get increasingly flirty vibes from M(late night Texts, long hugs, surprise kisses on the cheek, etc), and even though I wanted and should have told him off (or beat the shit out of him) - D would tell me not to, and that I should trust her transparency about it and just ignore him - so I went along with it. That was until I found out M was getting way too flirty with D and was driving her down and away from her life goals that she had been working so hard towards - after and incident where M confessed he still loved D and kissed her when she was in no position to resist (I was not there at the time, but her friends were and did nothing to help her), we talked about how D needed to stop interacting with M - She surprisingly agreed to that, and was realizing how toxic M was to her and her future. 

A month or so later I found out that they were still hanging out and doing things that might not have been cheating, but were definitely not good for our relationship and her mental health.

I confronted D about lying to me, and told her that she needs to stop seeing him or its over between us - to which, and I quote, she said "I can't decide which one of you I love more"... I told her things were over between us the moment I heard that.

Since then I've been getting texts from her friends calling me an asshole and saying how much they always hated me. I also learned that D has been hanging out with M to cope with apparently the worst thing to ever happen to her, which was me... I honestly feel like I made the right choice, but I'm starting to question myself.... so AITA?

TL;DR - I dumped my now ex because she continued hanging out with her friend who was her extremely abusive Ex, even after she promised she wouldn't.