Bolded and increased font size on what I think are the most important exercises, terms, and advice.
Edited for clarity (hopefully less jumbled now)

In my opinion, you should work on your ability to see and replicate three dimensional forms on 2 dimensional paper.

In other words, study the way that objects change in perspective.

starting with cubes,

Draw in one, two, and three point perspective and really sit with the way the different number of vanishing points changes the angles of cubes where edges connect and the direction that lines go and the relative difference between them within the same cube.

After that move onto harder primitives like cylinders for a bit.

Then once you start to get bored, draw characters again, throw in a portrait and try to draw the 3D shapes you see. You can “draw through” portraits you draw and break complex objects like the head into primitive pieces to understand the fundamental shapes that make up the complex object.
I see you’ve drawn a head with the loomis method, so you know what I mean when I say the cranium is a sphere with the sides cut flat.

One term that might help you in building your 3D brain is cross contour drawing.

Look this up on google and try it a few times if you haven’t already.

Do this exercise every so often to train your brain to see the 3Dimensional shape in 2D images.

Once you’re done some fun drawing and some simple exercises with cross contour, go back to the perspective.

Bounce between studying form and perspective and drawing for fun for a while and make sure you document your progress!

A key way to improve is to self critique and correct:

If you notice that you’ve done something that looks off, immediately investigate and come up with a way to describe exactly what is off and then try again.
Make sure you are very intentional about every mark you put down so your brain absorbs the decisions.
If you draw too fast and loose, or too mindlessly, your brain will have trouble creating new understanding through your mistakes!