>>384
The worst thing you can do right now is fall into the beg trap of grinding boxes and doing mindless exercises. You have a naturally good sense of form, and that appeal would totally be murdered if you train your hand to become stiff. Your art will look like shit if you decide to use guidelines.
Actually, the only structure you need to learn is of the head and face, and the hands. Those are delicate things with lots of curves and proportions in weird places. And even then, you should learn to do it by looking at real life references, DO NOT look at anything with a guideline, you already have a good sense of form, it's like you can actually materialize your drawing into clay and shape it in the 3d space, just focus on getting proportions right. And don't get all excited over that either, you have the ability but it's not "good", you just have it, and it's a basic skill most artists need that many many begs do not have and end up symbol drawing their way to shit. Do not, I repeat, do not snuff out that skill by grinding boxes.
Just keep drawing from references, preferably real life, and your sense of form will improve, and that by itself may even fix the unappealing look of your faces.
***
(I mean, I don't know what your sense of perspective is though. Some people have a good grasp on the body's form but are shit at drawing rooms. If you're shit with drawing larger perspective, grinding shapes can help.)