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!