Grinding will always fail with abstract mathematics because it distracts you from divergent thinking and spontaneous insights. The problem is that university courses are on a fixed schedule so people that need more time than allotted for this kind of study are not accommodated, which pushes a lot of people away from mathematics who could otherwise contribute.
That said, I tutor mathematics and I hear all day long "I can't do this" and it has never once been true. For the people I help it is usually a matter of finding how they learn best (visual, verbal, hands on...) and/or finding an appropriate metaphor. And yes, even despite this, they often struggle; but genuine struggle is a good reason to put in more hours. I don't dispute that.
Your prof's answer is the kind of answer that disgusts me. It's not teaching. In mathematics it is exceptionally questionable, because it is tantamount to expecting you to derive the whole of mathematics by yourself.
That's the introvert's version of "just be yourself, you're overcomplicating it" -- i.e. just draw on innate skills that I have without realizing and you don't.
In various subjects, particularly abstract mathematics, I had to grind heavily in order to keep up with the class. I attended every office hours and pattern matched heavily, studying into the wee hours. (I had tried your method and failed in previous semesters).
Other subjects I had no problem. Low-level operating systems, writing, literary deconstruction, chemistry, foreign languages, statistics: I had no issues studying a reasonable amount and having a good work-life balance.
Eventually I realized that I was never grokking things with mathematics beyond a certain point. I was in office hours with the professor, asking how I could generalize the concept to a new problem. He said "just use your critical thinking skills! That's what employers are looking for." I truly didn't have the ingenuity to do it. I really wanted to be good at it, but I knew I never would be.
I think you are forgetting about the areas in your life that you don't excel at, that you couldn't be good at no matter how hard you tried. Some people genuinely have to grind at some things in order to succeed. For me, it allowed me to realize that no matter how beneficial it would be to have a Math minor, it wasn't going to happen. I spent that time pursuing coursework I would benefit more from.