Snarf

Long story short, ive been going with this girl now for nearly a year. We went out for a few months long before this started and then got in touch again and started dating.
She got accepted into a college in USA and im in the UK. Shes doing the course for a year and will be back at Christmas for a while.
I asked her out officially, already knowing that she would be going away, i didnt care because I loved her so much and wanted to give this a shot as I thought she was worth it. She was the love of my life.

We made the most of the time we had left before she got on a jetplane. She has now been in USA for 4weeks.

Information came to me today that she had already kissed and slept with a few fellas in the first week she was there. I had noticed she wasnt paying me much attention as the phone calls and facebooks messages dried up a bit. (Her phone wont allow her to text me in the UK).
I got annoyed and said to her that if this was to work we have to be in regular communication as without the physical part of the relationship, we have nothing and its the road to splitsville.
I asked her last week if she was 100% committed to us, if not then we should finish it. She said she loved me and wanted to keep it going.

Now that Ive heard shes cheated Im heartbroken. I knew it would be tough, but I feel like **** that i know this. She had told me before she left that she wanted to marry me in the future. Its 100% true that she cheated on me. We agreed to stay loyal to each other, ive kept my part of the bargain.

How should I break up with her? What should I say?
There is no way Im staying with her, if shes done that over in USA shes more than likely to do it when she gets home.

Thanks folks,