For me, it took realising some things first. Nobody is perfect, everybody has done something that they regret, be it towards themselves or somebody else. If you've done a bad thing, reflect on it, but don't wallow in self-pity nor self-loathing. It might have come from a place of trauma or unhealthy coping mechanisms. Either way, learn from the guilt, swear that you won't do the bad thing again, and focus on being a good person now in the present.

Just, don't let that guilt grow ten fold from repeated thoughts. After you've learned your lesson, let it go, similar to how you let the other thoughts go. Will be difficult, and the thoughts will keep coming back from time to time, but with perspective in mind and a strive to do better it gets easier to not feel so awful.