I Hate This Part

Hey everyone; its Erica. I have a little story and I would like to know what you think about this situation.

So; my ex boyfriend and me went out for about five years. We weren't really friends before we started to go out but in the end; he's my best friend now. The only thing in the way; is HIS girlfriend.
I mean before he was going out with her; he was always all over me and didn't think it was wrong to it but after he started talking to her...that stupid slut...hes been different.
He thinks I'm a bad friend and I don't know how to not cheat on guys. But honestly; i only cheated on him because he did it back to me. WITH MY BEST FRIEND; who mind you isn't anymore.
Back to the real story; so we were suppose to chill yesterday January 11Th after i got out of work at 4pm but when I called him; he told me he was with his girl at his house.
The only thing wrong with that situation; was that he promised that he wouldn't bring her to MY HOUSE; well his house but its like Ive been with him for five years and Ive lived in that house and he just thinks its OK to hang out with her in his room cuddling on his bed with the door shut and no one else home. Worse of all though; he sleeps with her and I just loose it. I cant even sit in school thinking about how he could or even would do something to me like this. Lie to me about having her there and everything hes done yesterday is so uncalled for.


What do you think I should do about our friendship and possibly our relationship in the future?