My half cent on this, to me resentment is closely tied with anger. As soon as I get my anger out, with exercise or journaling, resentment will get out too.
Also, when playing the tape of all the hurt HE caused, all the promises he DIDN'T keep, how CRUEL he was with me, etc. I shift focus and think:
-of the hurt I caused. I hurt him too. He cried because of me, too. At least once.
-I have not kept important promises. The most important one, taking care of MYSELF. I have found I resent MYSELF for letting so many people hurt me, for not protecting my body and my heart.
-I have been cruel to many people too. I have done really bad, harmful, stupid things. Ultimately I have been cruel with others and with myself.
So, shifting focus, I get so much material that I CAN start working on, that the feelings related to "what others did to me" lose importance. I no longer feel like a victim, but an equal player on this madness called being human - although one that finally -out of pain- got tired of destruction.
I made the promise to myself, not to ever hurt someone else, and not to harm myself anymore, believing any other opinions about me, believing lies about who I really am. Part of not hurting myself anymore is refusing to carry any more resentment. The antidote for that IMHO is compassion.