I've developed and delivered courses for people just like you, and despite the well-meaning but misguided advice of people here, don't ride, ride, ride while you are learning.
Take your early sessions in short takes. 10 to 15 minutes at the most. You've already found 30 minutes is a big ask. You admit you are trying to cram all this learning into a short period. What is happening is that the stress of your frustrations and efforts is mounting layer upon layer, until you are so frazzled after 29 minutes, you can't do anything.
There are routines to riding a bike. They have to be learned.
If you are having trouble even balancing on the bike, have the shop where you volunteered remove the pedals, and lower the saddle so your feet are flat on the ground when sitting on it. Then start scooting using your feet. Graduate to a downhill slope when you can scoot 10 or 20 yards.
The most important thing to remember here is to look to where you want to go. DON'T look down, but choose a fixed point ahead of you on a wall or a tree, and aim for it. That helps no end in balance.
When you feel confidence that you can scoot downhill, and use the brakes to come to a stop (keep your fingers on the levels are all times but avoid grabbing them hard if you feel you are going off course, instead squeeze them gently), put the pedals back on.
You do need to get a little speed up here to help your balance. It takes special skill to balance a bike at 2 mph with your feet on your pedals, so you need to scoot along a little faster than that.
Then report back here on your progress.
Oh, and tuck the cuffs of you pants into your socks to keep them from getting caught on the chainring teeth, cranks or water bottle cages.