Welcome. I have several thoughts that JUMPED in my mind all at once reading your story (it's not too long, keep reading and you'll see many of us are long winded).
1. Share sight with roommates boyfriend.
(the more info he has the better job he'll do at "cleaning out his closet" and getting his life on track so he can stop being a cheater and start being true to himself)
2. Check out www.lovemore.com
. Read all of the definitions, explanations etc and then read a few of the magazine articles too. They are VERY educational and helpful.
3. Be open and honest iwth your roommate. Let her know that you are in love with her boyfriend. But you respect YOURSELF enough to know that loving relationships should not be started with lies-and on that basis have committed to yourself not to date men (or women) who aren't fully open and honest with their partners as well.
4. Commit to yourself and be true to the decision to be polyamorous respectfully and lovingly to all poly and mono people by NOT dating men or women NO MATTER your feelings for them-unless they are open and honest with their partners.
5. Research Radical Honesty and institute it in your life immediately and in ALL aspects of your love life with every person potentially involved in your love life. (i get that it may not be possible with coworkers or extended family-but try to include it as much as possible in all areas).
I'm sure there will be more great advice. That's my top 5 suggestions.