The best and most accurate practice exam is Powerprep from ETS, the makers of the GRE. Though there are only 2 Powerprep tests so you may not want to use them both.
Before I can really help, I'd need to know a little more about the types of questions you are missing or if you have a pacing issue or is it just lack of attention to detail - which happens to many students in test situations.
I'm biased here...but my company, Magoosh, offers hundreds of practice questions, and all math questions have video solutions. We don't have score just yet, but it's a great source of practice material for the exam. You can try it out for free.