You would think at 21 he could be more mature, help me?

He broke up with me saying it would be a long time before there was anyone else and that he just needed time to think but possibly wanted to work things out. He said he knows how much it hurts to have someone move on quickly and saw his first ex with a new guy within a month and he would not do that to me because I was special. Bullcrap, within 3 weeks he hooks up with some new girl which sent me into a rage. I feel embared for believing him and the thing is I trust very few people. I know I deserve better but I cant seem to let go of the anger and betrayal feeling...How do I? I just think at 21 we could be honest. Believe it or not I have always been honest with him even if it hurt so I expect the same. WHy and how do I let go of the anger, its not healthy to hold onto.