Hello everyone

I just started with studying for the GMAT a few weeks ago. The study material I use are the
MGMAT and OG13. It was my plan to review 1 book a week and complete all the problem solving sets including the problem solving set from the OG mentioned at the end of each
MGMAT book. I did a diagnostic test before I started and i really had a terrible score...it was even below 400!!! My goal is a 650 score...and I only have 3 months to study.
My problem is that when I review a
MGMAT I understand all the concepts...but when I practice with the questions in the OG I get stuck....I make so many mistakes

It is really hard to stay motivated this way

Most of the time I do understand the question but I just don't know how to tackle the problem.
What can I do now? Should I review the
MGMAT again? Should I buy additional study material? And is it realistic to get a 650 score within 3 months? I hope someone can give me some good advice

Sorry btw for any mistakes in this message...English is my second language