<human>: Give me the gist of the text in a nutshell

Seemed like a great relationship. Living together for the past year and a bit. 
I went to London for 3 months for work. Came back home half way through for a week to see him, all was well.
He came to London for the end of my trip then we flew back home to LA together.

Now a month after being back, with serious talks of our relationship, spending our lives together and moving into a new house together, he tells me he cheated on me in the time between when I came back to visit and when he came to London. Which would have only been a few weeks.
He says it was just once, feels terrible etc etc. he was drunk out with a bunch of guy friends (who I thought were my friends too, but who were ultimately encouraging him, although it is purely my BFs fault) and ended up sleeping with this girl who was an acquaintance he had met months before but never hung out with.

Not sure what to do now. I love him so much and I really thought I was going to spend the rest of my life with him. He says he'll do anything to make this work and wants to be with me and keep to the future plans we had made, but I don't know how to deal with this.

The other thing is that I kinda suspected that he had slept with this girl. 
At one point once we were both back in la, but before he had told me, we were driving and she started calling him and he quickly hung up. I knew it was weird and shady but he denied it and actually made me feel kinda bad for accusing him of being shady. Turns out I was right...

He also said that she had been calling him every week and threatening him, to tell me and everyone in the mutual group of friends. 
So a part of me feels like this was just him saving his ass before I found out from someone else.
<bot>: Boyfriend cheated on me while I was overseas in London. we live together and had made plans for the future. Not sure what to do.