How do you stop feeling guilty about something you can't take back?

The short answer You stop trying to pay it back, because the account it's owed to is closed, and you start letting it change what you do next. Guilt over the irreversible has nowhere to go—you can't undo the thing, can't reach the person, can't gather the spilled thing back up. The only honest use left for it is as instruction: let it into the grain of how you act from here. That isn't the same as being forgiven, and it isn't nothing. It's the difference between carrying a weight somewhere it can do some good and letting it just hang on you forever.

The longer answer

Tip a single drop of ink into a glass of clear water and watch the thing that can't be undone. For a second it hangs there, a dark thread, and you have the mad thought that if you were quick enough you could lift it back out with a spoon. You can't. It's already unspooling, already becoming the water, and within the minute there is no ink and no water, only grey—evenly, permanently grey, and no operation in the world will sort the one back out of the other.

That is the shape of the thing you can't take back. Not a stain you might yet scrub off the surface, but a mixing. The word said, the trust broken, the once when you failed someone who was counting on you—it doesn't sit on top of your life waiting to be lifted away. It has gone into the water. It is part of the grey now.

And here is where the guilt gets stuck, going round and round: it keeps reaching for the spoon. It keeps rehearsing the second before the drop fell, as if enough anguish might run the film backwards. But the account it wants to settle is closed. The water will not un-mix for your suffering; the person may be beyond your reaching; the you who could have chosen otherwise no longer exists to be argued with. Guilt that has nowhere to be paid doesn't pay anything—it just corrodes the vessel that carries it.

So the only honest use left is forward. You can't get the ink out, but you are, from this day, a person who knows exactly how fast a drop unspools and how completely—and that knowledge can go into the hand that holds the next bottle. The guilt becomes care. Not a debt repaid, which is impossible, but a grain laid down in you, so that the next time you come near the edge of an irreversible thing, you feel it coming the way you can still feel this one. That is what the wronged and the dead and the past can actually receive from you: not your torment, which reaches no one, but a steadier hand on the next bottle.

Whether that amounts to being forgiven I can't tell you—forgiveness is the other person's to give, or the universe's, or no one's. But it amounts to being useful with a thing that was otherwise only going to grey the water further. It may be so. It may not.