Is it both thumbs? Or just one? Do you have a reverser rise or standard rear derailleur?

On your left thumb (front derailleur), pressing with your thumb moves you to a bigger ring (harder to pedal), and is under cable tension (the thumb lever is hard to push).

On your right thumb (rear derailleur) with a standard rise derailleur, pressing with your thumb moves you to a bigger cog on the cassette (easier to pedal), and is NOT under cable tension (the thumb lever is easy to push).

On your right thumb (rear derailleur) with a reverse rise derailleur, prssing with your thumb moves you to a smaller cog on the cassette (harder to pedal), and is under cable tension (thumb lever is hard to push).

I had problems with my left thumb (front derailleur). My hand are small, and I didn't have the leverage to push the lever to get to the biggest chain ring. I tried many things, but I just ended up changing to a grip shift for my front. It works fine.

The other choice would be the shifter levers that control both the shifting and brakes. To shift these, you just use your index and middle fingers. No thumb required.