noun, verbal noun, の adjective
My behind regret is the regret from before, which is all regret. So this means regret, or repentance.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
You really don't have any regrets, do you?
If you do that, you'll only be tormented by regret for the rest of your life!
I regret how easily I got caught in her badger game.