Why Can't I Shut Off My Thinking?

Here I am, working on a new year. I have my plans for working on a "new" me, but I still can't help it---my mind seems to be working overtime. How do you shut your mind off from thinking about all the bad things? I don't like dwelling on it, but it seems to enter my every-day way of life. I might be having a great day when something I see on TV reminds me of what he did to me. Not only did he cheat, he stripped away my trust. Trust is a very important issue in a relationship. How did I let him do that to me? I just want to start off a new year with good things, and here I am letting the old and past come in? How do we shut off thinking about what's happened to us? Perhaps by replacing the old memories with bad? Yes, that might work, but it's still in the back of my mind just setting there until something comes along to spark the memories. When will the pain go away?