Well it seems that you are doing the correct preparation, but the information just might not be sticking. There could be plenty of reasons for this, but one that I can think of is that you need to give the GMAT your best attention. That means, don't study the GMAT after work, trying studying before work. Don't study when you're tired, study when you're alert. Don't study when your sick, study when you feel well... etc, etc... Make sure the GMAT is number 1.
Also, you need to cover your topics more in depth. You are obviously study good material, but you are not retaining this information. So I would recommend that you cover material more slowly, and reinforce each new thing you learn with practice problems. Make sure you don't move on until you KNOW that you have this topic down. Also, keep an
error log of your mistakes on practice problems, so you can later go back to those and revisit the questions you've missed.
The final advice I can give you is to not give up! It doens't sound like you will, but the GMAT can be quite challenging. In many cases it can take months and months and months over long periods of time to really start retaining this information. And if all else fails, you should really try to sign up for a professional GMAT course.
Good luck to you, and I hope that your future studies progress nicely.
Also, take a look at my post here:
10-tips-to-improve-your-score-app-with-no-extra-studying-124393.html?fl=home It might help you with some of the intangibles