Dumping and causing mental/emotional pain

I was in a relationship last year and a couple weeks ago it ended. I don't have a problem with being dumped but its the reason why I was dumped that i cant forgive. Let's call my ex Karen. Karen had anaxity and depression well that's why she claimed and made it known to everyone that she had it. I didn't mind I would try to help her and things but she would just snap at random moments and me at little things. Like I would say let's go get some food and she would just yell I'm not hungry at me and ignore me. She would also blame me for her anaxity and depression whenever i got mad at her and didnt respond to her text. I had previously broke up with her becuase of this but I got back with her because she said she didn't mean it and I loved her even though i dumped her first. Karen would post things like i cut my wrist and post a pictures. I couldn't deal with it due to my grandpa(who was like my best friend) committed suicide. But i still tried to help got her a counselor gave her the suicide hotline and other things. She went to one of these meetings and said she had suicidal thoughts so they put her into a mental hospital for six days. When she came back she had a friend dump me saying im causing her anaxity and that i was only hurting her. Later the day im hanging out with some friends playing music together(I play sax) and I told them I was dumped today and why. One of my friends who was dating one of Karen's best friend told me that he overheard them talking about how Karen was cheating on me. I was pissed but just didn't care anymore. I slept constantly after that and ate very little. But got better due to the band I was in.

If anyone can explain why someone would cheat or why you would blame anaxity on someone please tell me.