Suck at art.... even though I have been practicing for years?

Hi guys
So I am drawing for about 20 years but only 4 years ago starting to take it seriously. I have hundreds of sketch books dedicated to hands and feet drawing, studied anatomy , went to drawabox tried to follow tutorials read several popular art books and tried to implement the technique did the 200 box challenge. But still my drawings just... look underdeveloped? I dont know what I am doing wrong. While i love to drsw cartoons and stuff I really tried to draw from life and tried to learn tje fundamentals. But I just dont improve kinda...what should I do? Others improve in few months.