I hate to say it, but sometimes the only thing that will work in these situations is TIME. At some point, when you least expect it, you'll let whatever "it" is go. I've had this happen in my life many times. It's just a feeling that comes over me at some point where I have put the issue in perspective and move on.
Frankly, unless it's causing you health issues worrying or dwelling on it, sometimes it's not a bad thing for your heart and mind to dwell on something that's bothering you. Rather than distract yourself, allow yourself the time to work through it.
If it's something small, I love going for runs with music blasting until the only thing that I can think about is the run itself and nothing else. I've also come to view skating as a distraction (this was NOT the case during high school and earlier, when I was competitive and it was just another source of stress, but since I've sort of redefined it as something fun, I've learned to use it as a relaxation technique).
If it's something serious, I'm not sure how much distracting yourself can help--the issue is still there, you're just ignoring it, which is a fine temporary solution, but won't help too much in the long run. I agree with Bostonfan--let yourself think through it, especially the reason why you keep returning to that one thought, and try to let it go (or at least put less importance on it). I'm not one of those people who is fully capable of living without regrets (although I'm envious of those who can), but I find that this usually helps me.