How to improve painting skills?

The best way to learn to paint is to go study under a master painter, but if that seemes to expensive and out of reach try taking some workshops in the summer and go to any college and get a teacher, somebody that can creatique your work constructively. Without constructive criticism it is almost impossible to get better.

I'm not familiar with what you've bought to help you paint.....but think about this:

The problem here seems to be more in learning to SEE, and secondarily learning to handle the materials. I suggest you take a course that includes exercises like slow contour drawing, gesture drawing, and others that slow you down so that you are looking at things like negative space, how elements relate to each other, etc., rather than what they are in your mind. Have you heard of the book/course DRAWING ON THE RIGHT SIDE OF THE BRAIN, by Betty Edwards?

To paint it really helps to be able to draw well, which means that you are able to see in ways that don't bring in stereotypical memories, which possibly is where the "cartoonish" tendency comes from. You need to be able to see in the moment without assumptions about what you're looking at.

I hope this helps. A class might really help. If you can't take a class, get to the library and read. Nicholaides, THE NATURAL WAY TO DRAW is a complete college course, and about the best one there is. It requires some discipline to work through it, but if you do, you will get where you want to go.

It is true that there are artists who come to these abilities in seeing and handling the materials all on their own; if you want a shortcut, though, take the Drawing on the Right Side of the Brain course. You'll learn what you need to deal with proportion and a whole lot of other things.

To translate these things into paint, a course to learn to just handle the materials and start thinking about color, would really help you.

Good luck!

What kind of painting do you want to do? The kind of painting you choose to try will determine the approach you want to take. I guess though, if the artwork is coming out cartoonish, you may want to brush up on your drawing skills or color skills. Studying basic color theory could help with getting a more realistic, less cartoonish, grasp of natural color. And improving your drawing abilities can make it easier to create a realistic, less cartoonish image.

Practice and feel ;o)