I 20F and my ex 21M had been together since we were 14 and 15 (6 years) I was extremely close with his family and from what I thought we were a very healthy couple, we broke up late December due to him cheating on me with another girl. This was an emotional affair as it had been going on for 4 months. He had told her she loved him and they had been sleeping with each other for 4 months. I didn't live with my ex and so he was able to get away with it for so long until one night he went out with the boys and then went back to his parents house, my friend called me saying there was a girl at his house she found out due to snap maps. I was obviously so upset but I didn't say anything to him yet. I went to sleep feeling like sh*t and the next day I called the girl 21F and she told me everything she also told me she knew about me the whole time, another thing she said was that his mum saw them in his room together and didn't say anything.

Then I told my best friend 21F and she drove me to his parents house (he still didn't know that I knew he was cheating on me) when I got to his house I fully just said you've been cheating on me and he burst into tears he tried to lie and say he didn't do anything and there was no girl there but I called the girl right there and then and she exposed him. After that I blocked him. A few days went by and I heard nothing I was obviously very upset so I stayed at my parents house for the next few days. A couple days later I get a no caller id call and it's my ex he was trying to apologise etc but I wasn't having it. The next couple of weeks I would get messages from him, through out mutual friends and loads of miss calls.

A month later, there was one of our mutual friends 21st birthday party- he had been trying to contact me for the past month and I ignored it. Anyway at this party he barely spoke to me but he did try, I was pretty drunk and dancing on the dance floor with one of his friends, I didn't know he was watching but I kissed his bestfriend he didn't say a word to me or his bestfriend 20M but I knew he was annoyed. The next day I woke up to a paragraph from him saying he feels like I did that out of spite and I was being disrespectful and wrong, I never replied to the message but I feel like an asshole for doing this to him so AITA.