I don't want to be Catholic anymore. What can I do?
The short answer
The very question shows you recognize the Church's authority. Seen rightly, that authority is a comfort: Christ gave it to the Church so that we might be saved. Look closely at whatever you are questioning: what it means, why it exists, and why you are better off for it.
Asking this at all shows you recognize the Church's authority; otherwise, why ask? That is reassuring, because no one who recognizes and truly understands that authority would want to be "not Catholic."
Likely you question some teaching, discipline or law and want to trust your own authority over the Church's. But rightly understood, the Church's authority is comforting, because Jesus gave the Church its powers so that we might be saved. Lumen Gentium (18) explains that Christ instituted various ministries for the growth of God's people, and that those with sacred power serve their brethren so that all may freely and in an orderly way reach salvation.
Look seriously into what you are questioning: what it really means, why it exists, and ultimately why you are better off because of it.