Am i being to nice?!?

so i dated this guy for four months at usc and i took the next semester off so we broke it off. I would come visit and we would hook up and we were gonna be friends we still had feeling for each other. one weekend my friend told me he was dating my sorority sister for 2 weeks and he or she didnt tell me b/c they didn't know how, but he apologized and they really like each other it just happened and now 3 weeks later are official, me and him were together 4 months and never decided to make it official, bit i get it things happen sometimes you can't help liking someone but im still hurt over it of course, im not mean or cocky but i am sorta a 10 and her a 3 if you compared us but, he and she both are going through a smoking faze like all the time 24 7 and i don't smoke so maybe thats why he likes her so much because some girl likes to do it as much as him and he can always be hanging out with her now. I have been real stressed out in the last year and don't want anything to bother me or drama when i get back to usc and i live with her in the house so i told her i was happy for her and it shouldn't be awkward, i just need a clean slate in my life but it was hard to do and i think she maybe didn't deserve me being that nice about it, she didn't even really apologize and i found out wrong but what do i do when i see her i wanna be nice but i am not fake its gonna be to hard or ignore her which is hard to do? and i have to see him everyday next year also its hard acting like you don't know someone you care so much about but i don't know if its appropriate to be friends with him? or it'll end up killing me more inside?