I've been having trouble keeping my thumbs on top of the reins while riding? And suggestions to help me maybe fix that problem asap?

My trainer made me ride in wrist braces so I had no choice bc I have that same problem
When I first started riding I couldn't keep my hand closed and my trainer made me hold rocks
Try holding the reins normally and put a crop under your thumbs, your hands should be about 6 inches apart, and holding it there, also be sure your shoulders are flat against your back which can help
My instructor got me to hold a loose ring snaffle in my hands. I forced me to keep my thumbs up!