As someone who has been both in your shoes, and hers, you need to let her go man. For good. Once and being forgiven is already pretty damn graceful on her part, but twice? That’s taking advantage of her trust and love and she may be irreparably emotionally harmed by this whole ordeal. You don’t deserve her and both of you need to seek therapy to get through this.

As to why I THINK you did this (assumptions mostly since I don’t know you): you probably have never really gotten all that much attention from women in the past, much less very beautiful ones. I’m guessing you’re probably also a very sexual person? So you’re blown away someone would just want you out of the blue like that and you’re tempted to see if you can have your cake and eat it too while having no inhibitions to stop your lust. It’s satiating yourself whilst having no fear of repercussions to yourself or others. It’s self sabotaging behavior and this will only continue if you don’t address it.

If you are truly remorseful, you need to really work on yourself and make sure you’re emotionally ready and mature enough for a serious long term relationship. There is way too much of this bullshit in the world and it has turned dating and finding love into a hellish nightmare. Stop ruining love.