I suspect this is one of the reasons why writing gets harder as one gets better at it. (Unlike everything else in my life I've ever learned to do.)
It's because one's using more muscles, committing more energy--even as one is learning to make it look easy.
There's a bit of advice to writers that runs something like this: Don't hold anything back. Don't save anything for later. Shoot your wad. Use it up. Use it all.
Use your coolest ideas, in other words. Push yourself with your most challenging stories. Write the exciting bits now, not later. Don't hoard your stories, because the cleaner you scrape the bowl, the faster it refills with even better stuff, and that filler you packed in there to try to dole out the good ideas? It's just going to have to be cut anyway.
It works for me, anyway.
In other news, why am I already out of bed, showered, and drinking tea?