It doesn't matter what medium you choose (oil, watercolor, digital, etc.), the following tips should help anyone who wants to get maximum results with their medium.

1. It's not about knowing the limitations of your medium but rather how to work around them. For example, in watercolor you can't put a light color on top of a dark one (with some exceptions), so you need to learn how to plan out your light areas first. Know how to get certain effects with your medium. Getting a well blended gradient is not the same technique for each medium. The best thing to do is get a scrap paper or canvas and just try stuff. This makes it easier to learn rather than doing just "finished" paintings, because with a finished painting, you always go back to what you know works and what you have been doing in fear of ruining the painting.

2. Be honest, are you really struggling with oil paint or whatnot or is it your foundational skills in art that are the problem? I've seen people say "well, I'm not very good at oil painting," and I say "well, how good are you at color theory and composition in the first place." See, everyone has a certain "default" level of skill. What you've learned so far can be applied to ANY medium you want. You just need to then learn the specific techniques of that medium to master it, but if your foundational skills are not there, you will struggle with any medium no matter what it is. The three main foundational skills for painting are color theory, composition, and drawing. Mastery of these will make you a great painter once you finally get used to the techniques of oil painting or watercolor, or whatever your choice of medium happens to be.