They always say that you never know how you would react in an emergency situation, the same is true for cheating. I think for many many many people, including myself, it's listed on the top of the relationship deal breaker list. I mean who wants to stay in a relationship where they don't trust someone? But the thing that outraged me when my boyfriend told me about his cheating, was that somehow, I still loved him. And I kept thinking "what the hell is wrong with me? Why don't I just leave?" In real life it's not that simple. It's like that emergency situation, your feelings are still there, even if you desperately just want them to dissolve.

I'm really happy to here he's being open and supportive, right now it probably doesn't seem like much but it can be building block number 1. Yes you are kind of starting a new relationship and this one starts with your trust for him not at 0 but at -100.

"I can't think of any memory, even pleasent ones, without wondering how many girls he was emotionally intimate with that day"

Sometimes it helps if he can write out a timeline for you. With as much detail as you need and as accurate as he can remember (or look up if he still has emails to reference). This can help answer your questions and give you a framework to work with.

"But, I wonder how long it will be before he slips?"

By checking the accounts it will help decrease how often you think this. Watch his actions, look to see how he's changing to be a different person than the one who would slip.

"I feel like I can't share this with anyone because if we stay together I don't want to color their perception of him, so I feel very alone"

It is very isolating. I told one friend, and she was as shocked as I was. She was supportive but I could see her silently wondering why I was still with him? I had to be crazy. And after some time I just felt like I couldn't burden her with talking about it as frequently as I thought about it. It's great if your bf is there to listen. Don't hide your thoughts/feelings from him, let him be there to support you and give him the chance to prove that he's worth it.

It takes a long time, but it does slowly get better.