Hiya! I think it's great that you are trying to do lots of studies. That said, I think you might not be serving your best interests by your chosen method of practice. If you only want to learn how to paint eyes, then by all means continue. However, if you're interested in other stuff, then I'd suggest trying to get a more general knowledge base in place before doing those kinds of isolated and repetitive studies. I'd suggest first learning more about proportions and placement of those eyes on the face. Loomis is a great place to start and the ebooks are available for free. Another good one is "Drawing on the Right Side of the Brain" which is helpful if you want to learn how to "see".
In addition to that, though, you'd also be well served by drawing stuff that isn't from a 2D source. The trouble is that, when you study from photographs or other drawings, the image is already flattened! Half of the challenge of learning to draw is learning to establish a sense of depth. To do that, you need to draw the 3D world around you. Try doing some still lifes and drawing stuff around your room. Start learning the basics of perspective.
Progress might be slow at first, but I'm confident that you'll see improvement if you keep at it. Let me know if anything I've written wasn't clear. I'm happy to explain more!