Hi Chrissy,
I think that getting your thoughts organized in the form of a letter is a good idea, whether you end up giving her the letter or just talking to her. What you have here is a good start.
However, to improve your chances of a good outcome, you should definitely consider re-writing parts of your letter, and here's why. As it is phrased now, your letter seems to be anticipating a negative reaction from her. It also sounds almost like you are giving her an ultimatum -- either she can "put up with" all the things you plan to do, or she can leave. Now, I do understand that there is probably some truth to this, insofar as you have realized that you can't permanently suppress this part of yourself (nor would you wish to), so she will indeed have to decide to accept it if your relationship is to continue. But I don't think that it is a good idea to force that choice on her before she has even had a chance to process the information you are giving her. Instead, in the early days of talking about it, I would simply focus on helping her understand what you do and what it means to you. Let her make the next move, and don't shut down the communication process at this point by saying "I am going to do this, this and this whether you like it or not."
I would also give a little more attention to the part of the letter where you mention "sneaking around and hiding." Consider telling her that you realize you shouldn't have done that, that you were afraid of losing her if she found out, and that you now want to share all of yourself with her and be completely open and honest. Believe me, she will be more upset that you didn't trust her enough to tell her before than she is about the CDing itself, so give some thought to how you will deal with those feelings.
You are doing the right thing by preparing for this important talk, rather than letting it happen unexpectedly. I wish you all the best.
Love, Marla xx
