Living with your ex is definately going to be a difficult situation! I guess you live with other people too? From your post it seems you haven't started term yet - when you go back, throw yourself into your work, your societies, your other friends. By the second term, people aren't in as fixed as social groups as you think: just because certain people weren't your good friends before you left for xmas, doesn't mean they won't be in six months.
It's completely natural that you feel sad about the end of your relationship - you just have to concentrate on moving on. Remember that the relationship is over for a good reason. Oh, and don't beat yourself up about feeling sorry about it sometimes, it's completely natural and with time will fade. Hope you manage to work it all out.