I think that he has fooled himself in a way into thinking he is over this girl when he says "I finally forgot our first time somehow" but it is obvious that he hasn't forgotten anything when he recounts all the fun times he had with her. He doesn't want to be reminded of how great this girl is because he is afraid to slip back into her arms and thinks that if this happens only trouble will result.
I think that he has fooled himself in a way into thinking he is over this girl when he says "I finally forgot our first time somehow" but it is obvious that he hasn't forgotten anything when he recounts all the fun times he had with her. He doesn't want to be reminded of how great this girl is because he is afraid to slip back into her arms and thinks that if this happens only trouble will result.