As the Buddha says, you can’t go back and erase what you did by feeling really guilty. Just realize that the best that can be asked of a human being is to recognize a mistake, to resolve not to repeat it, and then actually carry through with that resolve.

"In the guilt cultures that we have, especially in monotheism, there’s the sense that justice has to be done one way or another. Whereas Buddhism doesn’t talk about justice at all — it talks about being skillful in your behavior. Now, sometimes skillfulness does involve punishing people who are wrong, but you’re not necessarily trying to get justice done.

Think of the case of Aṅgulimāla: all those people he’d killed, and yet the Buddha saw that he had a potential. So he taught him. Aṅgulimāla was able to give up his murderous ways and become an arahant. The karmic consequence of all the killing he’d done was simply that, as he was on his alms rounds, there were people who were probably upset that he’d literally gotten away with murder and they would throw things at him: sometimes tearing his robes, sometimes bashing his bowl, sometimes bashing his head. But as the Buddha said, that was a lot less than the consequences would have been if he hadn’t gained that attainment.

So the whole purpose of Buddhism is not to settle scores and to bring about justice, which is why the question of guilt doesn’t come in. Feelings of guilt are not encouraged. There’s a passage where the Buddha says that when you recognize that you’ve done something wrong, the proper response is: one, to recognize that it’s wrong; two, to resolve that you don’t want to repeat it; and then three, to develop lots of goodwill [mettā]. Develop all of the brahmavihāras, but start with goodwill: goodwill for the person you’ve wronged, goodwill for yourself, and then goodwill for everybody.

Goodwill for yourself is to remind yourself that by punishing yourself you don’t gain anything. Part of the psychology of guilt is the feeling that if you punish yourself a lot, then others who might want to punish you will hold back. So you can pre-empt them.

But from the Buddha’s point of view, karma doesn’t necessarily have to bring about justice. There are consequences, but there is an out: You practice the noble eightfold path, you gain awakening, and you don’t have to meet up with all the possible karmic consequences of what you’ve done in the past.

There’s that image the Buddha gives of throwing a hunk of salt into a river.

He starts out by saying that if everything bad you did had to be punished tit for tat, there would be no way anyone could gain awakening. But the actions you do will give results of the sort that they are. In other words, unskillful actions will give rise to pain; skillful actions will give rise to pleasure. But the pain will get greatly reduced as your mind gets more expansive — again, more expansive with goodwill.

Here the image is of throwing a large crystal of salt into a river of water. You can still drink the water because there’s so much more water than there’s salt. Whereas if you don’t have an expansive mindset, haven’t developed the mind in virtue and discernment, and you haven’t learned how not to be overcome by pleasure or pain, then your mind is like a small cup of water. You throw that large crystal of salt in, and you can’t drink the water. It’s too salty.

So regardless of what you’ve done in the past, you can develop your mind so that you can mitigate the results of past bad karma. Which means that the proper response is not guilt. As the Buddha says, you can’t go back and erase what you did by feeling really guilty. And it’s not a question of preventing punishment by punishing yourself. Instead, just realize that the best that can be asked of a human being is to recognize a mistake, to resolve not to repeat it, and then actually carry through with that resolve.

Thoughts of unlimited goodwill, compassion, empathetic joy, and equanimity all help to strengthen that resolve. So when you find your mind in a spiral of guilt, remind yourself that even though some ways of thinking would encourage that and tell you that it’s a good thing, but the Buddha’s way is not one of them. Because again, the question of justice doesn’t come up, the question of guilt doesn’t come up."

~ Thanissaro Bhikkhu "Guilt & Shame"