First of all, this type of exercise definitely isn't fun and the fact tat it looks like you filled pages upon pages speaks a lot to your dedication. I think you will achieve great results by just sticking to your current course. Art growth tends to sneak up on you and it's a wonderful feeling when you can see it one day. The simplest way to add depth to a purely line drawing is line weight. Thicker lines on the heavy and shadowed parts of the object, thinner where the light hits. You can also outline the entire object (except the light parts) to make them stand apart from each other on the page. I didn't do the whole page, but since your form is so good it was so much fun! I showed two types of using line weight, I hope you see what I mean. Don't give up! It will 100% start paying off and when it does, and you will see good results, you will feel on top of the world. (´▽`ʃ♡ƪ)
This was incredible advice. The difference is very stark. Thank you so much for all the time you spent to give me pointers it means the world!! :)

