From Chapter 7 ("Forgive Yourself") of Just One Thing by Rick Hanson, a useful taxonomy to apply to something you feel horrible about:
Sort what happened into three piles: moral faults, unskillfulness, and everything else. Moral faults deserve proportionate guilt, remorse, or shame, but unskillfulness calls for correction, no more. (This point is very important.)
So yes, some things are worth regret; other things really aren't. And when you've categorized events, take responsibility where appropriate, clarify and be relieved for what you're not responsible for, learn from the experience, make amends as needed, and then "actively forgive yourself".
(cf. Homer Simpson Career Advice: "You can't keep blaming yourself. Just blame yourself once, and move on.") - ^z - 2013-01-24