How do you forgive yourself for a terrible childhood mistake?

When I was about 15 years old, I was doing something that I regret very much today, it does not leave me alone ... It makes me feel like a bag of garbage. My whole problem now is that I'm a non-believer, even though I'm still looking for God, I feel hostility towards myself, I don't know how I can go on living, whether I can continue my life or not end my life.