Nikolai,
My advice to you is very different from some of the things that I've read in this thread. Maybe I'm the one who's wrong, so take these with the appropriate grains of salt:
1. Just keep drawing a lot, and the rest will follow. In his book "Chuck Amuck", animation director Chuck Jones writes that his art teacher at Choinard told the class that everyone has 100,000 BAD drawings in them, and the sooner they get it out of them the better it will be for everyone. In high school we wrote short essays which the teacher then examined, marking every error in red for correction. In college the professor told us that the whole approach was wrong, and that if we simply got into the habit of writing a lot every day, good grammar would naturally follow. Looking back, my college prof was right, because in high school we learned to hate writing, and in college we learned to love it. Often the only thing standing between us and what we want (weight loss, writing skill, drawing skill, whatever) is habit. Building a good habit is often difficult, but if you keep a good habit going, everything else usually falls into place easily. With rare exceptions, do NOT spend a lot of time on a single drawing. You're better off moving on to the next one. Make sure that you're having fun with it, and that you're doing it often.
2. Stop worrying about developing your own style. It will happen naturally even if you tried to stop it from happening. I mostly studied animation, and in that field one of the big challenges is to get everyone to use the same style, at least while they're on the job. If JJ's style melts your butter, pick out 50 or so of his best drawings, grab some tracing paper, and trace each one of them several times until you can draw a near-copy on your own without the tracing paper. Then start drawing slight variations of the drawings, and then finally some new poses of the same characters. You are NOT ripping anyone off, you're simply learning how to be a good animator! The development of your own style is inevitable with time. As long as you're having fun doing it, a tracing exercise like this will make you a better artist (especially if you want to be an animator).
3. Avoid being self-critical. Arrogance is bad, but for some reason many artists (including my daughter) get into the habit of badmouthing their own work because it isn't as good as they wanted it to be. I think that's a mistake. Kids do worse when they're constantly told that they're worthless, even if the kid doesn't believe the statements. Repeated sayings sink into parts of our brain, and can cause trouble even if the sayings are disbelieved at the conscious level. No matter how much you love drawing now, you'll erode your joy if you keep muttering that the work stinks. If you want to progress, you'll need that joy. Obviously some drawings are better than others, and nearly every drawing is not as good as the artist would like (even for experienced artists). That's just how the process works. Treat each drawing as a step forward on your life's journey -- including the dead-ends and mistakes. Make sure that you're enjoying what you're doing, or you'll inexorably learn to avoid it.
I could write more but it's getting late. I hope this was helpful.
--Will
William L. Dye ("willdye")
Software Enginerd
Lincoln, Nebraska (USA)