You are describing rumination. I used to have a big problem with it. Do you have a counselor or doctor? Please mention it to him or her, if you do. If you do not, you can make an appointment with a doctor or counselor and describe how this rumination is distressing to you. Finally, as hokey as it sounds, I have used this as a technique and found it very helpful. When I start thinking about Regretful event A, I deliberately interrupt my train of thought, say (inside my head) "You can stop thinking about that"…or "You don't need to think about that anymore" and deliberately turn my thoughts to something pleasant. I also imagine a portal to outer space (like a laundry chute, sort of) where I bundle the things I think are getting me down and that are past events I no longer need to feel regret over) and shove them out into space where they disintegrate. You are welcome to laugh at my technique but I have indeed found them helpful.

You might try my techniques once and say "darn,that didn't work" but keep using them and I think you will see some improvement. Courage! :-)