To add my two cents worth:
Don't dwell in the past.
It doesn't mean you can't learn from the past, and it doesn't mean you can't occasionally enjoy something old, and it doesn't mean that history is unimportant.
What it does mean is don't spend so much time thinking about past events that you sacrifice the present and the future. I see so many people worry so much that it destroys their life.
In addition, I also see too many people "keep record" of other people's wrongs and use them as ammunition in arguments. 99.99% of the time, the past event has nothing to do with the argument except to drag the other person down and prevent the other person from moving on in life.
Instead, learn from your mistakes and start with a clean slate. Your mistakes are in the past, let them stay there. The only thing you need to keep is the lesson learned from the event. Step back, take a look, ask yourself:
-How can I prevent this in the future?
-If I can't prevent it, how can I minimize its chances of happening again?
-If it does happen again, how can I minimize its negative effects?
Make mistakes into opportunities to learn. Don't beat yourself or others up over mistakes.
Edit: And oh, yeah, don't be afraid to spend time thinking about stuff. It's very tempting to make a quick judgment and not think something through. Grab some paper and a pencil for stuff that may be complex. Most of all, relax! Stress and anxiety can be big barriers to having a clear mind.
If you feel like walking around or engaging in some constructive physical activity, by all means do so. It helps push blood to the brain, which helps your thinking, and it helps channel anxiety and stress. Look for lakes to walk around and trails to walk on. Exercise is something nearly all of us need to do more often.