It sounds like you might not be solving problems in the most efficient way possible. For many GMAT problems (math especially, but verbal too) there is often a "long" way to solve a problem and a "short" way to solve a problem. The long way is typically the traditional set up equations/do lots of calculations type. The short way often involves seeing some sort of trick or pattern that you can use to answer the question more quickly/accurately.

So, my advice is to adjust your studying - rather than just doing the problems, really break them down and find the best way to solve each one. Many times, this is NOT the way that your book will tell you to solve it (especially the OG).

Finally, understand that it's not necessarily bad to have to guess on the last few questions (though yes, 10 is too many). If you're getting all your questions right up front, it won't hurt you too badly to guess on the last, let's say, five questions. Chances are that you'll get one right, and one will be an experimental questions that doesn't count. Also, look for opportunities in the test to guess on problems that you might spend 5 minutes on but get wrong, anyway - such as very complex math problems.

Good luck!

Thanks!