Leaving the past behind

Why is this so hard to do sometimes? I'm having a problem with it. It pops up every now and then and for some reason I can't make it go away. I even have dreams about it. I wake up and tell Joel, b/c it bugs me! I hate even thinking about this person. Not that I hate this person. (This has nothing to do with Joel, any friends or family.) I don't want to get in to it exactly, I think b/c I'm embarrassed that it still gets me. How do you let go of things that bother you? How do you find closure? It seems that some how this person keeps popping up. Not directly, but through other people. I thought today maybe I wasn't being truthful with my self about how I was feeling. Like that I was really ok, but I'm not and that's why I have these dreams and such.

I don't have a lot of things in my life that bother me. But this does. I replay it over and over in my head. 

How do I make it stop?