Writing a letter like this is very helpful. It gets you to put in writing what you really feel.
The actual content is not so important. The few elders who read the letter will skim-read after the first lines where you disassociate. It will not provoke them to think but will probably close the thinking process off completely. After that they will make an announcement that you are not Witnesses anymore, and the Cong. will assume you were either disfell. for immorality or jumped before you were pushed if any know you DA'd. The gossip will fill in all the gaps where the facts are not known.
Write it for yourself - you will feel better for doing it.