I think really everyone remembers happy things about their exes, because every relationship has its happy moments, even some of the abusive ones. (I'm not in any way saying that an abusive relationship is a happy one, definitely not, I'm just trying to say... most people are in a relationship because they're happy with the other person, at least at first.)
I certainly remember a lot of happy things about my relationship with my ex, despite the fact that I can definitely see my new boyfriend and I lasting a long, long time. I embrace those memories (my favourite one is when we were sitting in a park and he turned my face towards his and kissed me softly on the lips); why try to shun what's happened to you in the past? It's happened, and you should just enjoy the moments that the two of you had. I'll always have some feelings for my ex; that doesn't mean I'll ever, ever act on it. He treated me so well pre-breakup (which was mutual) and we're still friends, so of course I'll still have some feelings. But they're neither as strong or as insistent as they used to be.