This is a bit like one of my personal focus hacks. When I'm struggling, especially in the morning, I'll often set a five minute timer, and tell myself: "surely you can focus on something for five minutes." This is usually enough to get me over the hump, and in the mood to actually get stuff done.

I do think there's something that happens after a short period of intentional focus, where you cross a "barrier" into a state of mind that is more conducive to long-term focus. But there's definitely sometimes real resistance to crossing that barrier.