I have a suggestion which will help you retain concepts better.
Always learn a concept with the intention of teaching it to someone. When trying to understand the solution, ask yourself "How will I explain this solution to another person"
I applied this technique to improve my proficiency in the quant and verbal. This is how I did it:
1. After learning a concept, I go back to my room and pretend to take a class; as though there were imaginary students (after I while, I became a GMAT instructor and I got real students). If I can take the class smoothly, it means I have learned the concept thoroughly. But if I fumble and mess up my imaginary class, I go back and learn the concepts again.
2. To see if I have understood the solution to a problem, I explain the problem to my imaginary students. If I am able to do so, I know I have understood it. Otherwise, I would go back and look at the solution.
I have a very active imagination. So all this thing was happening in my mind. I didn't speak out anything. I just sat on a chair in my room and visualized the entire thing.
Some people might think I'm nuts... But this technique has worked for me

I request you to apply this and let me know if it is helpful.