Pbrow pretty much nailed it, but I just want to add that if you think that you can send a letter of disassociation and then continue to go to the meetings and it will take the pressure off then you are very much mistaken!
If you DA then the eyes will be on you 10 times more. The elders will only want to talk about you returning. Every time you turn up to a meeting will give everyone else a chance to show how holy they are by ignoring you. In short, it's only going to increase the pressure.
One last point: A letter of DA is still playing by their rules. What's the only way to win at Tic-Tac-Toe? The only way to win is not to play*. If you do want to stop associating with them then... just stop! You don't owe the organisation a single thing. You're not bound by their rules and you don't have to play their games.
*(with thanks to the film Wargames for the illustration)