First, you can't go back to your twenties and you shouldn't try. When you are still in a school environment, there's an environment that for doing problems for

First, you can't go back to your twenties and you shouldn't try. When you are still in a school environment, there's an environment that for doing problems for problem's sake. For all you have trained so far, be able to solve problems is how you were measured and feels like a life's purpose. By thirty you probably get the hint that life is not about solving fake problems, and most of the knowledge you learn at school is useless and pointless. If you try the suggestion to sit down and do exercises, I doubt you would be able to keep at it long enough for any gain. But at thirty, you have the luxury of not worry about midterms and finals and you probably can afford multiple books. So first, find your love of physics. Second, collect old text books. Third, read them. Fourth, criticize them. Fourth, throw away the book that you've digested or is bad. When you can throw away all the books (the knowledges are all online anyway), you are learned.