Well you said it. If you want to be good you'll have to put in a lot of hard work and be able to take harsh criticism (sometimes non-constructive criticism too). :-)
If you want to do this type of thing for a living you'll need to improve, and you definitely have a lot of room for improvment (to put it nicely).

Some basic pointers to get you going in the right direction:
1. Learn the fundamentals of art (non-anime/manga). This would include learning perspective (absolutely necessary for all artists, and especially manga artists as they will be doing plenty of backgrounds), composition, lighting/values, color theory, etc.
2. Learn human anatomy, inside and out. Quite literally. Memorize the size and placement of the bones and major muscles (names too, though that isn't as important). You need to know what's going on underneath the skin if you want to be able to draw people well. Once you understand human anatomy and can draw a well proportioned "real" person you'll be able to simplify them down to the anime/manga style.
3. Draw from real life. Sketch anything from random objects around the house to people to vehicles to architecture, working on accurate perspective and proportions initially. As you improve, you can start adding values (shading and highlighting). But a detailed drawing is still crap if the line work is faulty.
I could go on, but basically you'll need a much stronger understanding of the fundamentals of art if you want to be good. :-)
Hopefully that was motivating, not depressing.