When i use my thumb for a short amount of time the joint that bends gets really stiff and almost a numb feeling sometimes painful. how can i fix this?

? Trigger finger: This could be overuse. Trigger fingers are uncommon at your age, but overuse from gripping a pen or other object too tightly is very common. Have some one observe how you hold your pen while writing. If your fingers are blanching, you are gripping it too tight. Try a larger diameter pen and see if this helps. A hand therapist may be helpful as well.