Wouldn't everything I just said suggest that if I can feel the direction of gravity, it must be because I'm accelerating somehow? If I were blind and in free-fall, I wouldn't be able to tell what direction "down" even is, and yet, the muscles in my legs tell me very explicitly that they are "lifting" me.

The answer, or a very useful answer, anyway, is that something is accelerating: the ground. The stationary ground accelerates up through curved spacetime to push me. That's how I can tell.

But how does the ground accelerate without moving? Shouldn't the Earth expand if it's accelerating?

We're definitely approaching the limit of the things I can explain at the moment, but the key is time dilation. The fact that your head ages faster than your feet is the reason why the Earth doesn't expand. It accelerates without moving because time is also curved.

You can think of it this way: If you were to graph the position of objects through time, the Earth's surface might curve toward the y-axis, the time axis, because it is accelerating. But, the y-axis also curves "away" from the Earth's surface in the same way, such that they maintain their distance. And so, the Earth doesn't appear to move.

Isn't that cool?