- Study a little anatomy, but do not get too bogged down in "correct proportions". Few of us complies with the vitruvian man :D
- Study foreshortening
- Sketch a lot, fast, without looking too closely is the best practice.
You will discover new qualities. Do this with both references and
from your head or better: sketch people quickly when sitting on the
bus. The great thing about that, is that you hardly have any time,
and it will hone your abilities because it needs to be fast.
- Do not go over them immediately, give it at least a week before
you look at them.
If you look at some of the old masters, there will be technically things that are off. The classic example are The birth of Venus by Bottichelli, where the neck is too long (conveniently covered by hair), the arms do not add up with the shoulders and elbow, the feet seems too small.
Some of da Vincis drawings also look odd; his drawing of hands, they often seem too elongated with odd placement of knuckles.
But no one questions - and should not - question the value of these works. They are not smaller for it; in fact some of the art lies in the artistic freedom.
(I know from experience of drawing hands that things might look off, but are not. They human hand consists of an idiotic amount of bones and are notoriously tricky to draw. They almost always look wrong. And I have studied my hands for decades!). The human skeleton is a gangly thing. It helps to have a mental image of it, but do not get too focused on bones.
What you might want to consider: drawing is not really about photographic reproduction, we have cameras for that. The big question is what you want to express. The quality of a piece of art does not lie in how engineeringly correct it is.
- It is helpful to have a little anatomy knowledge but not too much.
- It is probably more helpful to study foreshortening.
- Draw stick-men reasonably in proportions.
Then, drawing from your head: try to exaggerate certain body shapes. Draw the same character as thin, chubby, fat, obese; toddler, teenager, adult, old. And there are practicals: it is easier to draw a face in profile or "half profile" (where the tip of the nose aligns-ish with the chin, see image below), than full en face.
It sounds to me that you are too critical too fast. Yes, if you drew The birth of Venus it would look off, for good reasons. You will never be 100% happy, no matter how good you get; that is in the nature of things.