To me, it sounds like it is not the subject itself, but the learning process you enjoy. When you get good at it, the challenge is gone, so you move to something else. Maybe, instead of just searching for new hobbies, the key is to find new interests or expand your knowledge on things you already enjoy.
Maybe take a class in music theory, production, or something. You already have an interest in it, and you are good at it. But there is always more to learn. Just something to keep you learning, and that learning becomes useful in your career.