OK, I’ll add my 2 cents, FWIW.
I was in a similar situation myself, but not with a roommate and over a much longer period of time. I’ll try to keep this short.
Tom and I had a history going back a few years- dated briefly, were friends, then later sorta-kinda dated again. Friends with benefits would be fairly accurate. From a lot of talks we had about this, it boiled down to- I liked him a lot, he liked me a lot, he was freaked out about his ex-girlfriend who completely betrayed him and he was afraid of a serious relationship. So we kinda-sorta dated, nothing exclusive.
I hear from a mutual friend that Tom has started (really) dating Katie. Katie is a friend of mine, who knew the situation and knew that I had feelings for Tom. Mutual friend basically forces me to call Katie, who sobbingly apologizes when I tell her I know about it. I say, don’t sweat it, I’m a little upset* but I don’t want to ruin a friendship for a guy.
So, a couple months go by. Katie, on more than one occasion, asks me for relationship advice with Tom. I try to be supportive and offer unbiased advice. Well, things don’t work out between the two of them.
Several months later I found myself hanging out with Tom again, and things happened. I fully intended to tell Katie myself, but a mutual friend (bitch, more like) found out and spilled the beans. Katie was pissed. She never spoke to me again.
I couldn’t understand it. She had broken up with him, she knew of our previous situation, was it that shocking that I might again pursue him? I felt that I had done the right thing by stepping aside and wishing the two of them the best (even though I was upset by the situation), but she couldn’t extend me the same courtesy? I knew that I should have told her myself (which I was planning to do but was slightly distracted by celebrating my 21st birthday, in fact the whole thing happened on my 21st birthday, I thought it could wait a day), something that she hadn’t even done for me (they had apparently been dating for a couple of weeks before the mutual friend decided that Katie wasn’t going to tell me and I needed to know). She wouldn’t hear any of it. Oh well, she sucks.
So that’s when I realized that while some people value friendship quite highly (yours truly being one of them) others do not. I have chosen my true friends carefully since then, and while I may have fewer, I know that petty disputes are not likely to destroy our friendship.
I second the advice of others. Make new friends, find distractions and reasons to get out of the apartment. Now that you know how your friend behaves, be prepared to find a new roommate or new apartment.
*Completely, totally pissed and heartbroken would be more accurate.