Ah, unrequited love for metamours! The bane of the poly world!
Maybe getting together with him while she is in the same apartment isn't the hottest idea ever in what comes to protecting your heart right now. Can he for example visit you?
Blogs that have the same problematics as yours and that I am familiar with are AnnabelMore's Story of a Secondary (similar dynamics to yours) and TeamD79's Life Alien to Our Own (pretty much exactly your situation in many ways).
"Resentment destroys more alcoholics than anything else. From it stems the root of our spiritual disease."
"In dealing with resentments, we set them on paper - list people, institutions and principles with whom you are angry. Ask yourself why you are angry."
"In most cases it was found out that our self-esteem, our pocketbooks, our ambitions, our personal relationships, including sex, were hurt or threatened. We were sore, burnt-up." Alcoholics Anonymous, 64-65.