I would add a sentence (or two) about the fact why you are writing a letter instead of "just" talking to her. That you are nervous to tell her, that you afraid how she would react, that you want to give her a bit time to "understand" what you just opened to her, that it is easier for you that way.....
I don't know how it started, and I can't remember the last time I haven't been this way. I want your support and your help to find out what makes me do this so I can stop hurting myself.