There is no karma. Stop blaming karma. And stop blaming yourself also.

Who are you blaming? The person who hurt someone long time ago doesn’t exist anymore, only in your memory. There’s a new person now, right here, right now, asking this question, who is experiencing guilt. How could it be the same person?

That’s all you know and that is all you will ever know. Forget about this other person which is a fiction now. Focus on this new person instead. So there’s guilt. It’s okay, it’s natural, everyone experiences it. Accept it. The sooner you accept it, the sooner it will pass. Don’t try to run away from it, it’s not possible. All you can do now is accept the emotional experience of guilt that is coming from the realization that you were hurtful long time ago. And of course, you will try not to do it in the future. But that you already know, otherwise you wouldn’t feel guilty. And there’s no guarantee either, all you can do is try.

That’s the best you can do - accept your emotional experience and know yourself better in the future for it. That’s how you grow. Don’t dwell on it or constantly re-analyze the past. Don’t try to figure out how it happened or what made you such a bad person. There is no past, it’s only memories. All you know is how you feel right now. It’s immediate, you can’t really argue with it.

That’s how you forgive yourself, by not expecting it to happen and not creating and reinforcing negative self-talk about someone who doesn’t exist anymore.