It is never too late to improve your score. GMAT is just a standerdized test. Most important thing you need to be aware of to get score is "Discipline" in your study habits. Remember, ETS doesn't test how many materials you have studied instead tests how well you understand the concepts. Hence there is really no point in spending all your time searching for new materials. My recommendation to you is that pick up any of the reputed book (PR, OFFICIAL GUIDE, KAPlan
etc)as a starter, learn the concepts to your bone. Identify you weak spots, focus all your energy on improving those. This forum is a great source for learning concepts. Definately actively participate in the discussions. Here are few pointers to make the best of your available time.
One rule of thumb is "If you can't explain it you haven't learnt it". So when you spend time in this forum try to actively participate in the discussions. Try to explain your reasonings.
Keep an error log
of all mistakes you do and review it periodically ( Once a weak ??)
Practice under timed conditions and review even the once you get right.
Full length tests only helps you to build up stamina, untill you get your concepts right avoid full length tests.
Online Challenges give you pleanty good questions to practice.
Make the best of this forum and good luck to your exam. Hope this helps.