I have good math days and bad math days; we all do. An outsider might think the good math days are the days when you have good ideas. That’s not how it works, at least for me. You have good ideas on the bad math days, too; but one at a time. You have an idea, you try it, you make some progress, it doesn’t work, your mind says “too bad.”
On the good math days, you have an idea, you try it, it doesn’t work, you click over to the next idea, you get over the obstacle that was blocking you, then you’re stuck again, you ask your mind “What’s the next thing to do?” you get the next idea, you take another step, and you just keep going.
You don’t feel smarter on the good math days. It’s not even momentum, exactly, because it’s not a feeling of speed. More like: the feeling of being a big, heavy, not very fast vehicle, with very large tires, that’s just going to keep on traveling, over a bump, across a ditch, through a river, continually and inexorably moving in a roughly fixed direction.