How do I deal with this?

I was suspicious of my boyfriend a few weeks ago and so I went through his email to see if there was anything I should be wary of (there wasn't.) We have been together for about a year now though and I just learned from emails from his past, that he was with his last girlfriend for 7.5 years (he is 25, I'm 21- so they were together from the time they were about 16 until ~24). He had told me that his longest relationship was about 5 years.

The problem is, his email had sent her letters outlining their entire love story, from the start until the finish and then some post-breakup "forgive me and take me backs." I read all of this and first of all, feel awful that I know wayyy too much about his last relationship and second of all, I can't stop comparing myself to her. In certain situations, well what would ___ have done? Am I doing this like she would have? What if she did something better than I do, that he might have liked better?

I don't want to tell him that I read all of this, but I did tell him that I feel really self-conscious about myself knowing that he was together with someone for a really long time before me.

My question is, how do I forget about the letters (I used to read them a lot, but I just deleted them from my computer) and how do I stop comparing everything I do to this girl that I have never met? How do I forget about her and stop letting her hinder me getting closer to my boyfriend? Any help is much appreciated!!