Download the Five Last Acts book in the resources thread, there's a great chapter on this. The now password-protected asher site also had an entire section on writing a good note, I haven't been able to find an archive of it yet, but it might be somewhere on here.
Remember that a note is not for you, it's for the living, just as funerals are. Focus on the good times you had with them. Don't blame them, don't insinuate that there was a deficiency or nonaction on their part that caused you to make this decision. It's important to not make promises or requests aside from factual ones related to your final affairs. Saying something like "please always be happy, don't be like me, please don't miss me" can cause a lot of guilt and make them regretful later in their lives when they're having a hard time.
Thank them, remind them you had an illness, this was a logical decision you made and they had no influence on it. Whether that's true or not doesn't matter, since the note is for *their* closure and not yours.
It's also helpful to have all your final affairs ready, so leave your passwords, insurance paperwork, bank info, etc ... so that hey don't have to relive losing you by doing it all themselves, and they can focus on grieving. Leave your desired funeral arrangements but try to be vague. Maybe you want no funeral and to be flushed down the toilet, but that may be very disrespectful to your family who may want to grieve with their loved ones (emphasis on their, maybe you have no friends/close family but their loved ones may want to come to support them). Leave basic info, any religious/other required rites if you have them, and maybe some money if you can.
There's nothing you can do now to heal them from that. But you can reduce their suffering and give them more time to grieve by being prepared and considerate.