I don’t want to be Catholic anymore, I want to officially be not-Catholic. What can I do?
The fact that you ask this question reveals your recognition of Church authority. Otherwise, why even ask the question? I find this reassuring in that no one who recognizes the Church’s authority, and really understands it, would ever want to be “not-Catholic”.
I suspect that you question some Church teaching, discipline, or canon law. You in essence, want to trust your own authority over that of the Church’s. But seen in its proper perspective, the Church’s authority is comforting because Jesus endowed the Church with its powers so that we might achieve salvation. Vatican II’s document Lumen Gentium (LG) explains, “For the nurturing and constant growth of the People of God, Christ the Lord instituted in His Church a variety of ministries, which work for the good of the whole body. For those ministers, who are endowed with sacred power, serve their brethren, so that all who are of the People of God, and therefore enjoy a true Christian dignity, working toward a common goal freely and in an orderly way, may arrive at salvation” (LG 18).
I suggest that you seriously look into what it is you are questioning, find out what it really means, why it exists, and ultimately, why you’re better off because it.