Regret.

I suppose everyone has regrets (if we're truly living).
Of course, it's easy to look back and see what you could have done differently (hindsight's 20-20 and all that).
It's not so easy to look at the present and scry all of the outcomes and options (though some of us try anyway).
When we're in the moment we generally don't write up the pro's and con's of every possible move, and then confer with a panel of ethical peers before making a decision (unless you're Donald Trump... Money can buy that kind of surety).
We do what we do and it's not until we've lived through the consequences do we realize how we could have done things differently (read: better).
This is the worst torture because you can't change it; you can't take it back (only in dreams).
All you can do is comb the wreckage for things worth keeping, learn from it, and let your past influence your future (change IS possible).
Besides, what's the point of history if you don't learn from it (and have the courage to try again)?
Regrets, when coupled with hope, can build our future (if we're truly living).

"Wisdom comes with winters." - Oscar Wilde