<human>: Can you summarize the given text in a few sentences?

I've been dating my girlfriend for a little over two years now, and a few days ago she asked me about some specifics about my past.  I didn't want to lie to her so I told her the truth, and she was upset about a friend I had a one time thing with years ago.

There was nothing there and both of us agreed that we should just be friends instead of any sort of romance at all, and all this happened years before I even met my girlfriend.  Honestly it's kind of awkward for me to even think about at this point.

That being said, I know she's really upset about it. I've tried reassuring her that I love her and no one else, that one time thing was a very long time ago but she still is acting withdrawn from me.  When I try to talk to her about it, she brushes me off saying she's ok but it bothers her. 

 I love this girl dearly, but don't know what to do. I feel guilty for something that happened years ago, which is ridiculous. I've been trying to reassure her, telling her that it was in the past and didn't amount to anything then, but she still seems really upset about it all. She tells me she's also embarrassed that she's upset. What can I do to stop my stupid past from harming my relationship?
<bot>: my girlfriend of a little over 2 years is upset because she found out about a one time date/fling with a friend years before i knew her. I reassure her that I love her but she's still upset.