I have very few regrets. I regret selling my '64 Volvo, but that was years ago. I regret trading my 1966 Fender Jazzmaster electric guitar for a Martin 0018 acoustic, but that was a long time ago, too. I regret selling one of my boats, and miss it more than I like to admit. But these are things; they are replaceable with something different but similar, and do not matter a whit in the greater scheme of life.
There are other things that I could regret. I might regret not spending more fatherly time with my children. I might regret not pursuing a career in performance more energetically. I might regret getting angry at inappropriate times. But in all these cases (and many others), I am still at it; still trying to improve--still learning. I'm not sure it's possible to regret something when I haven't finished trying to make it what I want. There is always the opportunity to make things right.
I suspect that the only things that actually warrant regret are those things that we have not really done our best at, and can not redo. The risk is that as we grow older and look back, the more things we can see that might lead to regret. The older we are, the more chance we have to view certain experiences as mistakes and thus to allow regrets to pile up; to allow fatalism to overshadow optimism.
Now there is a new(ish) Tom Rush song in which the following line appears: "We only grow old replacing hope with regret." It is a powerful reminder to always do our best while we can; to never give ourselves to the luxury of self-pity. And to be able to say "I have lived a good life and I have no regrets," and to be able to say it long before we are on the edge of dying.
Perhaps this is the fountain of youth.