*Clears Throat*

Wishing you had or hadn't done something.

When you feel sorrow for things you might have done, And no hope for tomorrow in the setting of the sun-
And the ocean is howling for things that might have been!

When you literally have nightmares about a bad choice you made -- as in the time many years ago when you took the wrong fork in the road and even now to this day, you're still lost.

disappointment in one's own actions or lack thereof

its wishing you could have lived through your past differently

that you would have done things differently

Wasting 15 years of life on someone

When you do a certain activity so much that you've hurt yourself...and you still can't stop doing it.