I see you already know which textbooks you need, so what I have to add is this:
Learn different ways of drawing. Dont go by just what one person says. If its too hard to draw a certian way then thats not the right way for you. Theres tons of approaches so just keep learning. The more techniques you learn the more you can combine to make your own or come up with your own. Basically take different approaches and find what suits you best. Practice and refine. You already are off to a great start.
Don't try to eliminate your mistakes... work on making them work for you.
Don't Fight Line... Use It
A bad drawing is evidence that you are trying something new.
Whether you try to get better or not is up to you.
Getting better is not in the end result its in the trying.
When your looking at a bad drawing and still have the
will to get better at that moment you are overcoming defeat;
a stumbling block which most would giveup.
The will to contine is getting better, and you not only
passed that stumbling block but passed a mediocre artist.
Also this little video helps out too: