I guess she must have been his first love since that kid she wrote the letter to was her first love unless the author has decided that didn't count somehow.
If that's the case then I guess he was hanging onto the letter just because it was his only way to get social contact with her, even though it's a really terrible way, and then in the end he probably did something really stupid, obnoxious and self-destructive because he was lousy at being honest and he'd spent too long being the blackmailing asshole to come clean. And now, all these years later, he apparently hasn't learned a damn thing.
Damn it, I know they're gonna be together in the end, but I sure hope she at least makes him suffer a bit first.
(Of course maybe all the little glimpses are deliberately misleading and he didn't do anything too terrible--certainly the whole thing is more complicated than it looked at first)