Blades,
Just so you know, you are still a member of the Catholic Church. Valid Christian baptism creates an indelible mark on the soul which cannot ever be removed by anything we or anyone else does. That is why the Church has accepted deathbed reconciliations from once-avowed athiests, murderers, Mafiosi, etc.
The idea that writing a letter could terminate one's affiliation with a religious organization arises from the Watchtower's own view of their identity: they baptize people as a sign of their dedication to God and membership in his organization.
The Catholic church has always viewed it very differently. This is the key to understanding the Catholic view of non-Catholic Christians. Long ago, when heretics had divided Christians into schismatic groups, the question arose as to whether or not persons who were baptized by heretics, (like Montanists, for example) were actually truly baptized. Yes, said the Bishops. Anyone who is baptized in the manner prescribed by Jesus ("in the name of the Father, and of the Son and of the Holy Spirit) was indeed a true member of the church, "born of water and the Spirit", to use Jesus' words. That is why non-catholics are viewed as "separated brethren" rather than members of a different religion or of some other Christian church.
Since Catholics believe and proclaim in their creed that there is only "one" holy, catholic and apostolic church, all persons who are validly baptized are members of the same Catholic church, even if they are not in full communion with the main body of that church's members, for there is only one body of Christ. There can be no other. So as regards your membership in the Catholic church, your letter was just as valid as would be a letter you had written saying that you had decided not to be born.
Tom Didn't see your post first, StAnn. We agree.