Why am I still hurting?

A couple months ago, my boyfriend dumped me. We were skiing with some friends and he asked another girl out, before dumping me. He said he had "forgot" that he had a girlfriend. I was so hurt. It's been a couple months, but I still miss him. I've tried to move on, but it still hurts. Whenever I hear "our" song, I always get sad. He meant a lot to me and he told me he loved me, but he dumped me? Why? Was I not a good girlfriend?