When you have really f*cked up with your g/f, how do you make it up/fix it?

Details withheld, but imagine you have royally F’d up things with your g/f because of something incredibly stupid you did. You’re ashamed of it, you feel like the huge a**hole you are, and you’re sad that you’re looking at losing someone who means the world to you.

But you can’t take back your actions.

Other than apologizing, what do you do?