Should I marry her or breakup with her?

Hi everyone,

I'm very much in love with this girl. We are currently planning our wedding for December. We've been together for 2 years.

Last week, I happened to read her chat log with one of her friends. She used my computer and forgot to log out.

She still loves her ex-boyfriend of 6 years whom she met when she was 20. He broke up with her 3 years ago. In the chat, she mentioned she'd always dreamt of marrying her ex while they were together. In her eyes, he was a perfect person. She'd waited 6 years for him to marry her, but for some unknown reason, he broke up with her.

When her friend asked her what she thinks about marrying me. She said I'm just an ok person. In the chat, she said in her own words, "The person I am still in love with 100% doesn't love me back. Nothing I can do. This guy is ok. I'm marrying him because he loves me and cares for me."

Now i'm heart-broken. I'm comtemplating of breaking up with her, but I love her too much now that I think breaking up with her is hard for me to do.

What should I do?