I just had an Aha moment.
I was reading an older post from Michelle Bennett's blog A Singer's Life, and she talked about the act of anticipation - keeping your mind one step ahead of your body while singing, or in my case, playing.
That's exactly what I have been experiencing in the moment when I am able to play my bits of piano practice at a desired metronome marking.
And by extension, what I need to learn how to do on my cello. I'm thinking that part of the solution is to play things slowly until my brain can get ahead of my fingers. That's a lot longer than I usually do, and also not something I have paid much attention to. I wonder if that may be part of the key to being able to play "more difficult" music.
It feels really cool.