make it sound like you were suffering a lot, very tired of it and can't really bear trying to live like this for the rest of your life.. have them know that you know it will be super painful for them to have you gone but that they won't have to worry about you suffering any more.. make sure they know that there was nothing they could have done and not to blame them selves for any thing, and that you had already tried every thing you can think of for a long time and it just wasn't working.. so it's not like you didn't super try to keep powering through all of this already..
i don't know how you want to make the tone though.. you can still say all of those things in different tones..... it can sound like you are depressed and sad about it, or the tone could be cavalier like "meh, this life isn't doing it for me and so i'm out of here" kind of tone, or just what ever you want i guess.. but i think the more sad and depressing you seem in the letter the more that sadness will kind of rub off on them.. although you might not want to outright just lie about it if you really are suffering and depressed.... so it's all up to you, but those are just some things to think about..
oh, and tell them that we all die eventually, you are just exiting out a little bit early, and that even though it seems far away, before too long they will be able to see you again on the other side.. this separation is not permanent, but only temporary.. tell them you will be watching over them and waiting for them at the rainbow bridge.. and that when they think of you, not to think of you in the sad depressed state that you were in, but to think of you as you are now...... free from suffering, whole again, radiating light, or how ever you want to put it (plus i don't even know what your beliefs are or their's, but i fully believe in spirit world and that we are still perfectly conscious after death)..
you can buy some colored pens if you want to write it (or different parts) in certain colors or what ever you want to do.. maybe draw some hearts on it too in a few places.. I would type the whole thing out on the computer first though and then when you write it down on to paper be meticulous and slow in writing it on paper so you don't make too many mistakes and have to scribble out a word that you messed up..
you're a cool person..
love and light..