Wait, why aren't you going to give this to her?
Kyle? Is that you?
Because I'm a coward, obviously.
Forget the letter. Track her down and tell her face to face. Preferably in the wedding chapel before she marries that rich, handsome prick.
Dude. I know this is easy for me to say, cause it's not my letter, but I would send it to her...she may never respond, but you tell a girl something that deep and heartfelt, she can't help but feel badly, yet good about herself for helping you learn to love. I dunno, I guess I'm saying I feel for ya, and I'd say just suck it up and send it in the mail, you'd be amazed how at peace it may make you feel.
From experience, here are the possible outcomes:
1. He tells her and she returns his feelings and they live happily ever after.
2. He tells her and she returns his feelings, but can't be with him because she's moved on, so they'll long for each other but never have each other.
3. He tells her and she doesn't return his feelings at all, but is touched by his letter.
4. He tells he and she doesn't return his feelings and is annoyed by his letter.
5. He tells her and she doesn't return his feelings but her boyfriend finds the letter, assumes she does, and leaves her, ruining her currently happy life.
So, you have a 1 out of 5 shot at being happy and a 4 out of shot of being not so happy.
Its not that simple. If it was a win-win situation, sure, he should send it. But its not. She's happy now and I'm sure he doesn't want to risk that.