If it's not too late, I have a suggestion that might set up a bit of a safety net for you. Obviously, you know her best and you probably have chosen to say things that you think she would be comfortable reading, but...
Maybe you should take out the part where you talk about being with her. Just that couple of words.
If you take that out, then if she doesn't recipricate your feelings, you can pass the letter off as an expression of your non-romantic love, and your relationship won't be affected in a negative way.
Just an idea.