FI's in dog house (and might never get out)

Sorry but I really got to vent here. I feel like the past serveral weeks of caring for my FI was to waste. I even went over to TnT to get some support and all he gave me is this 'I'm going to die' speech. Well... now that's here home with me, I found out an interesting tidbit about my 'beloved' FI.

To make a long story short, I found out my FI, the man I am planning to marry and standing behind him throughout his health crisis, was cheating on me while in California. I found out in the worst possible way. While unpacking his stuff, I saw a note and read it. It was an email 'this person' wrote, telling him how much she missed his (you know what) and that she hopes he gets better soon. I did confronted him and he admited to what happened. I just told him to pack his things and go back to Long Island to his mother's first thing in the morning because I do not want to see him or touch him right now.

So this morning he left my house here in PA, asking me to forgive him and telling me he appriciates me being there for him. I didn't want to hear it. I felt I was putting so much energy to make sure he is okay so we can start our lives together, only to see that being shattered. I even went down there to convince him to get treatment here in the US because I love him and know he will be okay.

Now I just don't know what to do now. He wasn't being happy with me, why would he cheat on me like that? I really love him but don't want a cheater. After what I have done for him, I felt that was a huge slap to my face.