From what I know, hypnotherapy would be perfect for such a problem. Since you consciously want to stop thinking about the past, but you subconsciously feel addicted to this form of worry, hypnotherapy could be a great way to resolve the conflict, and unite your conscious wants with your subconscious wants. Depending on the severity of the condition, it can also serve to find the root of the pattern of thinking.
Everyone worries, but here is a quick exercise to monitor and possibly curb such habits: Get out a piece of paper (or windows paint) and draw a circle, label it “Influence,” then around that circle, draw a bigger one, label it “concern.” Now brainstorm on a couple of things that you may have been concerned with in the last few minutes, and decide if your concern is inside or beyond your circle of influence.
Most of the time, the things we worry about the most are the things which we have no influence over; This pattern feeds itself, because no amount of worrying ever assuages the problem (though, it might seem to be accomplishing something). So, the key to effective decision making is in making sure our focus stays within that which we can influence, the goal being to keep your circles of concern and influence the same size.
This exercise is about 2 pages of a 350 page book called, The 7 Habits of Highly Effective People. I recommend it, and I hope this raises a little awareness.