Here's the thing, "drawing styles" are usually made up on exaggerating or simplificating things. In cartoons you get usually exaggerations: big noses, big hands, big heads; while in manga you get simplifications (or stylizations) of things, small or no noses; look at the mouths, they're usually just a line, no upper lip, maybe they will add the lower lip or no lower lip at all.
But all of these things are based on something, a realistic human figure. I once heard a pro artist saying that if you want to draw cartoon, manga, whatever, you should first learn to draw a realistic figure, why?, because of what I stated above, once you know how everything works you can twitch things up: stylize the figure for a more manga-like approach, exaggerate things for more of a cartoony look.
Real life drawing won't do you any harm, on the contrary, it will help you tons.
Another thing, it's hard, nearly impossible to improve without references, you need to know how things look/work to translate them into "paper". And I'm not saying that you always need to copy a photograph to draw a good pose. I'm saying that you need to observe (a lot) and gather everything you can, how a girl body changes when she's sitting, how a body turns when pitching in a baseball game. Practice drawing from photographs, draw your own hands, your feet, draw your brother. Man drawing from real life it's great coz you have the references in front of your eyes (literally).
Real life it's the base. 8_8