How can I stop doing this?

Cut your hands off.

Don't worry, I used to do the same thing. Just get another habit. My new one is chewing the inside of my lip. Its not as noticeable as your thumbs, but it can still hurt sometimes. Either way, its better han picking at your thumbs.

Try keeping a band-aid over it или something like that. If that doesn't work, try getting acrylic nails - it sounds silly but they'll inhibit Ты from picking.

When you're sitting on the диван, мягкий уголок или in a chair put you're hands under you're bottom, it works.

Woah, this is scary...I was just picking the side of my thumb before Чтение this...I can't stop myself:P

I have that same problem, and I've yet to find a solution to it, either :I

My hand is a dolphin!!

Drink blood.

Everyone has habbits.

Find something else to do when Ты feel like picking there.

one word: DUCKTAPE