Dear John, first of all I agree with inky that it is more horrifying that you cut yourself than kissed someone else, and I’m sure your girlfriend would think so too! Please do not ever harm yourself again.
I understand you feel very guilty but realize that nobody is perfect. Everybody makes mistakes. You do, I do, your girlfriend does, everybody. If everyone was perfect life would be pretty boring!
You didn’t intend to hurt your girlfriend, you were a little drunk and slipped up. It’s not the end of the world… forgive yourself.
By forgiving yourself you might think “oh well if I forgive myself for this, that means I’m condoning this bad behavior”… not true.
It shows more strength to forgive than to hold a grudge and keep beating yourself up about this. Try to accept the fact that you made a mistake, it’s okay to feel bad about it for a little, but learn to let it go and be grateful.
Hope this helped.