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I think something else to keep in mind is that you need to work everyday for at least 1 or 2 hours, but more is better.
I make tons of mistakes and every time I do I try and ask myself why did I make it, what went wrong and then I study that, try to improve one aspect of my skills at a time. Keeping this attitude so far helped me a lot more than reading specific books.
Also you can always set up a simple still life and copy that to improve your understanding of lines and values. Something as simple as your computer mouse or a wrinkled t-shirt. It helps a lot, just make sure you're actually drawing what you're seeing, not what you think you're seeing.
Not sure if someone covered this or not but im to lazy to read everything...
if your frustrated looking at others work you have to realise that they have most likely been doing it years longer than you have the main point here is to look at work for motivation or ideas as to how to go about your own, dont compare it to your own because 1) its not you and 2) the difference is in the experience you have.
just keep track of your work by holding onto your old drawings, trust me on this one when you look at you old stuff it wil make you want to laugh (or slightly throw up) and it will knock some sense into you when you get that little bug in your ear that will tell you everything that is wrong with your work when in actuallity your progressing.
and with the studies try to vary it up a bit, dont just study through observation alone every now and then take a step back and attempt to recall those objects, forms and shapes that you spent so much time on and draw and manipulate them. its good to work your brain a bit if your just copying everything down it will eventually become frustrating unless your some type of half cyborg freak. any way hope that was helpful i have a tendecy of rambling on to much.
one more thing-a-ma-jig dont ever fear f'ing up if you dont stumble here and there your clearly not challenging yourself enough and the more you walk away from a challenge the more you put yourself in a corner and you eventually crack and give up like a postal worker with an automatic.