How to evaluate and improve from practice tests? I'm confused.

Hey! So, I recently took a practice test for the first time and got a 1790. I am most definitely not happy with that score and I’m looking to improve. I’ll be giving the old SAT in January 2016 so I really have to do well as I won’t be able to give it again.

But my main question is how to evaluate the practice tests? I mean, I understand that the answer I chose was wrong but I can’t seem to figure out why or how the correct answer is the correct answer. And the whole point of taking practice tests is to know where you went wrong and learn from it so you don’t repeat the same mistake in the future.
So, basically, how do I evaluate and improve because I’ve taken several tests, timed, and seem to be stuck in the same range with no improvement?

This applies for both CR and Maths.