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.

I think you got the value and shape on the pupil and iris pretty good.You also rendered the eyelash pretty well. My biggest critique is the fact that I think you made the left upper eyelid a bit too light.I recommend darkening them to atleast the same value as the Iris.(Here's an example by Nicolai Fechin where he darkens the side of the eyes,if that help)

Your drawing is excellent; you did well in your lay in and shading. If I may suggest, I think the eyebrow needs to be a little darker. Best regards to you.

Hi @Cristina Kaiden, nice drawings! In the top right study, it feels like the eyeball is a flattened egg shape, and the iris feels a little small For me it helps a lot to lightly indicate the circle for the eyeball in the beginning. That way I can make sure that it's the right shape and the iris the right size. I hope this was helpful :)

Very nice! Remember to shade the eyeball like a ball.

Hi @Warren Bonett, Nice work! I'll try to look for things to help you further: Some of these appear a little flat. I haven't seen the reference photos but I think showing more of the thickness of the eyelids would help with that. I also find it helpful to start my drawings by indicating the sphere of the eyeball. Wrapping the eyelids around it correctly, like the equator line on a globe can give more depth and being aware of the spherical form helps a lot in shading; the eye is basically shaded like a sphere. I attached some images from a critique I did to another student, Tobias Höglund, whom I gave similar advice. I hope this was helpful :)

these are good though! I especially like that glassy eyeball goodness in the bottom right! Great one!

Good job on locking the eyes on the same plane. The bottom of the eyes are lined up great but check the top eye lids also and see if they are on the same plane. Looks like the far side eye is bigger than the eye that is closest to us. I dig that nose by the way. Great job

Self critique woman's eye: iris to large, too wide. Proportions in general need measuring to reduce "stretch"