I (26M) read a letter my girlfriend's (26F) ex-BF (26?M) wrote to her

My girlfriend and I have been together for about 2.5 years. I am totally in love with her. She's the best partner I've ever had by a mile. We'd both had a few previous partners, but she had one relationship in particular that was especially long/significant. He was basically her high school sweetheart, and then they kept dating into college. I think they were together for around four years or so.

Anyway, she was out of the house, and I was looking for something in her desk, and I (very much unintentionally) found this letter her ex had written to her. (I know these scenarios are all over this sub, but I'd like to make clear that while this kind of snooping is of course not great, I don't actually feel too guilty about it; my GF has often said that she likes being an open book and she doesn't really mind if I go through her stuff. I feel the same way towards her. I told her about the letter as soon as she got home and she was not upset at all. The only reason I feel a little guilty is because I obviously didn't have her ex's permission to read it). I've never really been much of a jealous type, and I was curious, so I read it; I didn't think too much harm could come from it.

But... oh boy, this letter. He wrote it to her when they were in the process of breaking up (a slow process of both of them gradually realizing that things had run their course, as she's told it anyway) and it is... I have never read anything so moving, so honest, that cuts so close to the heart of life (outside of, like, a James Baldwin novel or something). He speaks so eloquently about love and life and the search for meaning, about the two of them as people and their place in the world. I was gobsmacked. This probably sounds melodramatic, but I sort of feel like reading this letter has changed the way I feel about life.

And now I feel caught in a very weird place. I feel like this letter is hanging over my entire relationship. (This happened about a week ago.) I told my GF I had read the letter but I did not say anything about how much it affected me. I don't really feel jealous per se, but knowing that my girlfriend received *this* letter and still let the relationship slip away, it's kind of stunning to me. And as crazy as I feel saying this, it kind of makes me see her in a new light. Just a little bit.

But the bigger and much more confusing feeling I've been dealing with is... I think of that letter, and I can't help but feel that *that's* what I want-- the person who could make our shared lives into something so meaningful. I know very little about this guy, and I've never met him, but it has honestly shaken my world a bit to know that a person like this exists. I love my girlfriend with all my heart-- I really do, and I don't want that fact to get lost in the fray here-- but I could never have imagined someone who spoke of love like this before.

I really need some advice on how to handle this. I have no interest in compromising my relationship with my girlfriend in any way, but right now I'm having a lot of trouble conceiving of a way to move forward from this. It's kind of been consuming my thoughts. How can I regain the reigns on my heart?

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tl;dr: I read a letter my girlfriend's ex-BF wrote her, and I found it so moving that it's making me see my life in a different light.