Hi !
To be honest it's getting better bits by bits, which is good :)
Overall you lack of focus, what I mean by that, you seems to be panicked on your 15s sketches because you might not knowing what to do in this short amount of time.
I will suggest you to keep it simple, leave some anatomical shapes behind, like Alex Woo does, it will help you to gain confidence in your line : 
For your longer sketches, you seems to have few notions of perspective BUT your drawings are clouded by that huge weakness. You could put any details, if your foundations aren't stable, everything falls apart no matter what. I'd like to share an exemple of what you could focus on for 5 mins :


Basically it's getting a mental representation, to visualize before making any mark on the paper, so you don't waste time and have fun.
Still, don't overlook perspective before studying anatomy, because it's crutial and this topic is too complex for any beginners. I made that mistake in the past :)
Good luck!