you draw better than i did at 13 and i had about 10 years of attempting art before that, so if you give it some time you can improve, i promise.
One thing you need to work on is giving attention to your lines, rather than making out single stroked shapes and lines, try sketching them out and experiment with how thick your lines are.
I'm very glad to see that you aren't trying to dive into anime like most kids your age do, try making your own style its alot more satisfying.
Also study anatomy, search it up on Google or whatever or just look at some action figures (which is what i do)
study the way the anatomy of humans animals etc work and look and try mimicking it as well as you can or even making your own twist to it.
Dont get frustrated, art takes time and if you take what the people here in the NG art forum say then you will improve fast, this is the best environment for learning about art Ive ever seen, its harsh but its really good for you.