seeing them now in these small scale in the comment section, I like them a little bit more.

You really improved from first to second. I am not very experienced but I think to invest in your next drawing more time for accuracy in the proportions would help the likeness. For example the ear is to big and around the mouth something looks off. Also my images improved (in my opinion) from more consistent tones of the shadows. The more even and less smudged it is the more polished it looks for my taste. Best regards

I feel exhausted and struggle to sit through a few hour drawing session as well. Take a break, get up and do some chores, make a snack, go for a walk, do some yoga, etc. It's also like any other form of endurance: your body will get stronger and used to it over time (even though you think it's just your brain and arm, how can it be that hard? It really is the same as a strength building routine - I say this as someone who works out and draws/paints haha). If you're using a regular writing pencil, there is a limit to how dark those go. You can come in and emphasize the darkest regions with a ballpoint pen or marker (ballpoint pen is going to give you more ability for shading than a black marker which is going to only give you a flat black tone). I noticed that the eyes seem too wide open, and maybe a bit big, in all of these. I think that is our brain's natural tendency to 'normalize' or 'iconize' things. E.g., our brains subconsciously like to think of an eye as an icon of an eye, so when you draw one you may think about what you're drawing more than observing the actual abstract shapes. Try not to think of an eye (or any body part) as that body part, instead focus on the abstract shapes and angles that they form. I resized & traced over the photograph to give you a comparison.