Why has he developed feelings for someone else?

I have been in a long term committed relationship for 8 years. Since I was 15 and he 16. Needless to say neither of us had any meaningful past experience, and we’ve never had a “break”. We had lived together full time for 4 years when we decided it would be best for him to move across the country, to be with his friends and find work as he was unemployed here. Everything was running smoothly, I went and saw him for 3 weeks at christmas, he came home for 4 months (Feb-May) while he was layed off, and has now only been gone for just over two months. The only difference between this leave and the last is that he is now living with his best friend who just got out of a long term relationship.

Out of the blue, about 2 weeks ago, I noticed him starting to become distant, not wanting to talk to me on the phone. He said he felt like every time we talked we would just end up fighting lately. Finally, a week ago I confronted him about it, I knew something more than a fight was up and his response was that I could do better, and that he had just been thinking that our relationship had gotten complacent, that he wanted me to think about it for a few days and then we would talk so that we wouldn’t argue. Two days later, he still wouldn’t talk to me on the phone, but I forced some answers out of him via text. He said he loved me very much but wasn’t sure if he was still in love with me. I asked him if he wanted to break up and he said he thought so, that it didn’t mean that things wouldn’t work out for us in the end, but for right now it was probably for the best. That he valued me as a friend and we would always be friends, but just didn’t know to what extent we would be together in the future. He cannot call me, he says he’s not ready.

The next day we talked for over an hour on msn, I had a gut feeling that there was someone else and asked him to be 100% honest with me. He said that yes there was someone else, she was 2 years older than him and that he had just met her a few weeks ago and had been speaking with her about her own relationship and her feelings of complacency, and that he thought it paralleled our relationship. She then left her boyfriend, and slept with mine a week ago, knowing full well about me. He told me he might be in love with her. I asked him if he didn’t want to try to work things out between us, and he said he did, but not right now. Before this happened, he told me he loved me every single night, has always maintained a sexual interest in me, and has never had a wondering eye. I dont believe he went out in search of something, it was just a connected that happened because they shared some mutual friends. My question is this: can he really move on so fast and be in love with someone else while at the same time not over me, or has this conversation with the other girl just confused him and placed questions in his head? I believe that he has an emotional connection with her because of a similar experience and the fact that I could not be there emotionally with him the past couple months, but he admitted he did not find her attractive at first, that “she grew on him”. Can you fool yourself into being in love to justify your indiscretion? Thanks