I doubt you'll be hated but it's difficult to know what her reaction will be. And you won't know unless you let her know. I think I'd avoid the letter thing though. You need to be more open and talk to her about it.
In my experience one gets a less favourable reaction if the relationship has been going on so long that she thinks of you as an established friend, rather than someone exciting she has just met. But you never know.
Since I don't think she'll hate you I suspect the worst outcome is a rejection, she'll be a little more wary of you, and you'll be upset and waste time wondering if she'll change her mind, rather than moving on as you should.