I was in the same boat as you about 10 years ago! In high school, I fancied myself one of the best artists in the school, but I did realistic figure drawing and portraits. My ego was burned when other people drew manga and it was beautiful, but they couldn't draw much of anything else. Arrogantly, I thought, if those non-artists can do it, I easily can! That was not the case at all... Since then though, I tried really hard to draw manga, as it became a matter of pride (what a silly reason, but I'm happy as it introduced me to a whole new world)
I did what others have suggested, with continual life drawing and anatomy studies (this is huge, because many manga artists skew their anatomy, and if you don't know what you're doing, what to skew and what not to skew, it can look really wonky). But what I found that helped me most was redrawing full manga pages. This helped me draw and understand the less interesting parts of a manga (not just cool poses, but also dialogue scenes, etc.), understand comic pages as a whole, and that great way the figure interacts with sound effects or the background. If things still look weird, I would trace things until I got the hang of it. Often I would unknowingly make assumptions, like where a nose would go or how it would be angled, and my results would look incorrect. When I trace, it makes me focus on breaking those assumptions because I can literally see where the placements would go, and my own mind can't get in the way. Doing that enough I would eventually understand and intuitively do it on my own. Many people poo-poo tracing, but I think it can be an extremely useful resource, if used correctly.
As mentioned, figure drawing and anatomy studies are crazy helpful. You may not get the relevance at first, because real-life bodies don't actually look like manga bodies (especially when the life drawing models are extremely different body types and ages, and manga is often beautiful, young, and feminine people). But as you understand the mechanics of life drawing, when you make choices in your manga you will do so intuitively and it will look correct. If you can't make it out to a life drawing class, draw people you see around you on the street or bus (clothed, hopefully!) and the Internet has some great resources, both of clothed and nude models (Line-of-Action, QuickPoses, and SketchDaily are a few).
For instance, your image has many of the proper mechanics, and the design of your eyes are quite beautiful. But your arms seem a little stiff and straight, the skin of the top arm isn't reacting with the surface of the other arm. It seems like you have a grasp on fabric, I like your treatment of the shirt. You could benefit from varying line weights too. Thicker lines help suggest weight, which could help stabilize that arm, and thinner lines, such as the fabric wrinkles, are great for those small details. Pay attention to those kinds of things too, when studying other work. You have a lot of character and personality in your drawing, so great job there!
As you study several different manga styles, you will probably make choices about your favourite way to approach things, and before long you will have developed your own style! This is what happened with me, anyway. I had a very Natsuki Takaya style that evolved to more Katsura Hoshino, then became rather Disney-fied, and then became a whole muddle of different styles that I could call my own. Except for now, I've greatly simplified my look and strayed rather far into rubber-hose craziness.
But everyone has said it, practise makes a world of difference. If you're not drawing everyday, you're doing something wrong