I realized that most people (including me) sometimes fail to understand some maths notions because they don't try enough to understand them. What I mean is that some notions are complicated, really complicated, and often people with good will go straight on doing exercises, hoping that somehow, that will make them understand. So my advice is: dont rush. Try to understand the core principles. There are theorems, try to understand how to prove them. Will the theorems still hold if you remove some hypothesis? What can happen?

Of course the way of learning I suggest is not easy or fast, especially for someone who is busy with a full time job, but I guarantee that if you do it you will build for yourself some solid grounds, and you will feel yourself more intelligent.

Out of interest, I am curious why you need to become better at maths.