Hang in there Pete!
I started on a 36" wide board that was 130 liters
And still, I fell at least 100 times the first day

Here's why we fall at first:
As kids, we can wobble all over the place and never fall because we are climbing trees, running all over the place, skateboarding.. All the things that keep our small twitch muscles in shape.
As adults we walk on flat floors, flat parking lots and never do anything that requires us to activate our small twitch muscles, so they go dormant.
As SUPsters, we re-awaken these small muscles in our feet and calves.
Our bodies relearn how to balance properly when faced with wobbling around on the water.
Hint: Want a quick way to relax on your board?
Smile ??
You will be amazed how much a smile relaxes your body. while a grimace will tighten you up.
As mentioned by others, standing with your feet closer to the handle is actually more stable.
a wide stance will exert maximum pressure to the outside rail, causing it to sink lower in the water.
Volume on the rails wants to bob quickly back to the surface, which throws your weight to the opposite rail and drives THAT rail under..
The result is a LOT of big "corrections" are needed.
Like a car "oversteering" on ice..... the skids get bigger and bigger until you crash.
Feet close to the middle will lessen the amount of wobble, by reducing the leverage you have to push a rail too far underwater when you "correct" to either side.
You can't apply too much pressure to the outside rail, so this limits the amount of wobble.
Also:
Feet in parallel stance only give "side to side" stability
Adopt a "semi-surf stance" when practicing on flatwater
What this does is add more 'front to back" stability
I'll include several pics from my hat cam, that show where my feet are from different perspectives
the standard "Parallel stance" gives good "side to side" stability
In the surf, a "modified surf stance" will add "front to back" stability,
by giving you the ability to lean forward and backward to roll over waves,
and makes it easier to lean forward when paddling for a wave
Enjoy the struggles.... you are learning something that will change your life!
Yewwwwwwww