My girlfriend cheated and is begging for forgiveness...

About a month ago my girlfriend of almost 2 years broke up with me because she didn't think that we were 'compatible'. We're both 18 and we dated for a big chunk of high school. When she broke up with me I was absolutely devastated, especially since 2 days before this I had my wisdom teeth removed and she stayed at my house for days up until I got better, and it was honestly the happiest I have been with her in ages. We had been thinking about going on a 'break' all summer since we had been fighting frequently, but after those days she had stayed with me while I was recovering from surgery I saw her in an entire new light, which is why I was so hurt by her unexpected break up.

For the next 24 hours after that I was a mess, and the following night I BEGGED to have her back, and I told her that I wanted another chance and that I knew I could make things better if she let me try.

She then confessed that she slept with some other dude months earlier (beginning of the summer, roughly...) so I left her house devastated (Understand that me and her had been talking about marriage and kids, and she constantly told me that I was the only one for her EVER..) The guy was apparently some older stranger at a party she went to that I was also invited to, but couldn't go. They talked that night and that night only. Her reason for cheating was because at the time we weren't doing so well as a couple.

So I ended it there, and since then she's been begging to have me back, her latest attempt being last night. This hurts...I want her back but I know that the trust will never be there again. It's ironic because about a week before she broke up with me we had got in a fight because I didn't trust her, and she thought I was crazy for not trusting her..

She sends me diary entries that show how much she misses me, and she's about to start counseling apparently.

I told her last night that she needs to be over me, and that we need to be permanently separated. Did I do the right thing? She's devastated, and I think I kind of am too, but it's all just a big blur of feeling bad for her...I don't know what to do or think.

We were having relationship issues before I knew she cheated, and we were thinking about a 'break' before all of this too. She's my first long time girlfriend...what do I do?
My girlfriend cheated and is begging for forgiveness...
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