My Ex-GF cheated on me with another guy that I don't know who he is until this day. I found out by myself and ofc, she first told me that she didn't cheat on me that he just forced a kiss and she told him to back off that she had a boyfriend.

By my evidence, they did more than that, she went all the way and even told him that she wasn't regretting anything. I don't know if he knows that I was her BF at that time.

After having the talk with her, I went full on dread game and she initiated contact just after 4 days and was overwhelmed by regret and wanted me back. She even bought me expensive stuff.

Here is were I failed miserably. After a fuck session that extended itself along 3 days, I took her back and gave her a second chance but I told her that any little evidence or mistake done by her would result in a break up and I promised her that if that would happen, I would tell her dad about it.

Well, she couldn't handle the attention by the other guy and still kept contact with him. Her sister told me about it (she has been awesome) and I confronted her, she couldn't deny it and I told her dad.

Her dad sided with me and even said that she was a b*tch that, if she kept that attitude in life, wouldn't go far with it.

The thing is, she is now with the other dude as far as I know (her sisters boyfriend is a buddy of mine) and it's harder to swallow then I first thought. Yesterday she picked up her sister at my place were I was chilling with friends of mine and her sister. By what I know, she picked her up and didn't even ask about me, something she always did before.

Today I slept really really bad, always imagining her with the other guy and it felt upsetting.

What I would really want to happen (and I had the opportunity, didn't I gave her a second chance) would be, she reengaging contact and telling that she was sorry for me to tell her to fuck off. My ego is somewhat shattered and it's as if I can't move on for now.

Is there anything that I can do right NOW to feel better? I hate losing and knowing that she got rewarded for such bad behavior really drives me crazy.

Should I find out who he is and try to attained my goal to have some peace of mind?

TL;DR: Got cheated on, gave her a second chance, she fucked it up, my buddys GF is her sister and he tells me that she is seeing the other guy, I'm feeling bad. Want her to come back so I can reject her and I want to know who the guy is. But should I?