The great news is that you've barely scratched the surface of what you can do. In a nutshell, you need to do, and most importantly, learn from, the questions in the ETS books. Don't do any more
Magoosh / Manhattan / Barron's / etc.
Study ETS questions until you can:
1. define the question type
2. see what skills or concepts are being tested
3. explain the best method to solve
4. identify why the wrong answers are wrong and what defends the right answer (for verbal questions)
5. determine what can be learned from the question