When I was 12 (I'm 19 now) I made a terrible mistake.

I hurt the one person that meant the most to me horribly, I pushed her away just because I didn't know how to deal with my feelings for her, I insulted her badly and ended our friendship that was so important to me. 

It's been about 5 years since I last texted her and apologized, we haven't talked since. I can't stop thinking about it. I want to explain why I behaved that way back then and that it wasn't her fault. But I don't want to cause even more pain to her by suddenly contacting her again, it's selfish. I just want her forgiveness because I can't forgive myself. I'm a terrible person.

How can I ever truly forgive myself and get over it? I don't want to think about it anymore, there is nothing I can do to change the past. I have to deal with the fact that she's probably a different person now anyway, and that she's not a part of my life anymore and probably never will be again. 

Any advice?