Focus on one thing to improve on at a time (like colors, proportions, perspective, etc.), and judge your improvement based on if you were able to accomplish that rather than the whole drawing being "good". It's a lot of trial and error in my experience, and it helps if you like experimentation.

Here is an example of using the method above: Only focusing on drawing the proportions of the human face correctly, and studying different ways to get proportions right (such as the grid method or measuring the distances between each feature.) This can also train your eye to see what is working or not working in past and future pieces.

Some resources: For color and lighting I recommend Marco Bucci and James Gurney on YouTube! I think it's something that isn't focused on as much as it should be.

also ngl caffeine helps with motivation for me but that won't work for everyone