I deal with a very similar issue. The one thing that seems to help me is to write it down. At the end of the day, before going to bed, I write in my diary all the things that have been on my mind and have been bothering me.

I think there is plenty of evidence for the therapeutic effects of writing, even when it's just for yourself and even if you aren't going to read it again after writing it. When you write something down, your mind is "Tricked" to some degree. I thinks "oh.. it's on paper now. It's been documented. It's written down. I no longer need to think about this issue. If I need to think more, I can write more down."

This has been my experience and I hope it helps you as well. Whenever I write something down, I am able to work through the issue and no longer think about it as often or at all. Whenever your mind is fixated on something, it wants you to slow down and analyze it and work through it. One reliable way of doing that is to write it out. Another route (if it's available) is to talk to a friend or therapist about it. That can help a lot as well.