I have been dating this guy on and off for 6 years. Recently we got engaged. When we first met he told me of a close female friend he had but she moved away for college. According to him their relationship was never the same. Years past and she gets married and divorced unfortunately last year. I just moved in with him and I had to use the computer and found a letter that she wrote him 11/09 saying how he told her that he loved her he wanted to take care of her and be with her always. The letter was long and she told him she just wanted to be friends. During this time we were dating. And now I'm hurt and confused.

He's obviously not over his ex and you've his rebound or consolation prize.

If you're not comfortable with that arrangement, then walk away. No need to force yourself to be in a relationship where you're not the other person's priority.

I agree with I Wish that he has not been honest and violated trust is a monumental hurdle to overcome.

At the same time , so is violation of privacy. I don't want to sound too harsh, but... as a rule you don't find too much you're not looking for on a computer.
I suspect you did a bit of snooping.

It is time for some serious communication between you two if there is any hope of saving this relationship if you feel it is worth saving.

I wish you well.

Not a lot of trust here, and if you look hard enough, sometimes you find something that you wish you never found. But you did, so now you have to deal with it. I agree with the other opinions here, but I am going to be harsh. Sorry. Nothing to be confused about. You are his second choice. He would be with this other girl if she was willing. Do you want to be with this man knowing this? Of course not. Dump him, because he doesn't love you..

Its clear that this guy of yours is not serious with you my dear, move on with life and get yourself a guy who is serious remember they alwys say the first cut or love is the deepest so I thnk your guy still loves her... sorry if I told you the hard way but that's the truth