I've recently geared waaaay down to 58 gear inches.

Spin, spin, spin.

Along with gearing down, I had a professional fitting on my bike and some spin coaching.

As a result of all the above, I find myself spinning at rates I never thought possible.

As part of my spin improvement, I have learned to "stay ahead" of the pedals in downhill situations, and thus avoid "bobbing."

On a few long downhills, I have found myself fatiguing as I attempt to stay ahead of the pedals, and as I fatigue the pedals start to catch up with me and drive my feet.
When this happens, I feel a brief sense of anxiety, fear, panic, whatever.
I think this comes from a sense of the bike driving me instead of me driving the bike, and my body interprets this as a loss of control.

Because of my ultra-low gearing, I can easily convert from staying ahead of the spin to slowing the spin by putting more of my weight on the pedals.
As soon as I convert from staying ahead to slowing the spin, my anxiety disappears.
I only experience the anxiety when the bike catches up with me and starts to drive me instead of the other way around.

It might help if I share my method of staying ahead of the pedals.

In order to spin at super high rates going downhill, I transfer more and more weight to the saddle by literally pulling up on the pedals, as if trying to raise the bottom bracket.
At the same time, I make an image in my mind of trying to hit the handlebars with my knees.
Of course, I can't hit the handlebars with my knees, but creating the mental image of wanting to do so helps.
My physical therapist refers to this as creating organizational intent.

When I envision my knees going towards the handlebars, it moves the organizational intent out of my thighs and into my hips.
I also envision pushing my feet over the top and I attempt to pull earlier in the spin than I think necessary or possible; and all of this contributes to putting more weight on the saddle and pulling up on the bottom bracket.

Using the above method, I usually hit some sort of aerodynamic terminal velocity before the pedals start driving my feet.
However, with a tail wind, sometimes my pedals catch up with my feet and start driving them; and, similarly, with a long, long hill, I sometimes run out of steam and the pedals catch up with my feet and drive them.

When that happens, I take a new tack and revert to slowing by back pedaling.
I put my weight on my pedals, especially the rising pedal, and resist it as much as I gracefully can without mashing on the descent portion of the spin.

So, I put weight on the pedals, with the majority of my weight on the rising pedals, and this slows me down to a speed where I feel in control again.
This involves a whole leg-and-hip movement, where my hips rock but the center of my pelvis does not go up or down.

I don't like it when the bike pushes me instead of me pushing the bike.
When the pedals start driving my feet because I can't stay ahead of them any longer, whether because of a tailwind or because of the length of the hill, I regain control by reverting to a back-pedaling, weight-on-pedals, weight-off-saddle mode.

Like I said, this only happens at extremely high spin rates that I previously thought impossible.
I can see how one could transfer this to lower spin rates where one feels the pedals driving the feet.

The original poster might consider gearing way down for a few days and playing with converting back and forth between staying ahead of the spin and back-pedaling braking.

Fun stuff.