I think you've done a great job with your letter so far. It's from the heart, it's telling your parents what you truly feel, and it's respectful. Every parent who loves their kids wants to know what is in their hearts.
Even so, this is going to be a hard letter for your parents to get. Be prepared for that. You mention some things in your letter that you might want to go into more detail with your parents on in person, after they read what you've written. For example, you wrote:
Well if you where to ask me if i believed in God at any age up until about 14 i would have probably said yes because any other option seemed ridiculous and could only originate from the devil,
Your parents will probably want to know why you don't believe in God now. If you do believe in God, they will want to know why you don't believe that the JWs have the "truth." I'm sure you have solid reasons, and I think it's important for you to be able to tell your parents those reasons.
Another thing you mention several times is that you feel insecure, different, like the black sheep. And while it's a good thing to acknowledge that, don't let your parents point to your insecurity or your young age as proof that they are right and you are misled. You know you are doing what is right for you. Let me repeat myself: You know you are doing what is right for you! This is a very scary place to be in, but make sure you stand up for yourself. You know that this will be painful for your parents... you are trying to minimize the damage by writing them this letter. But you do not need to apologize for doing what is right for you.
Good luck with this, you are unbelievably brave to be going through this by yourself. I wish I'd had the guts to do what you're doing when I was your age.
Hugs!
GGG
