What exercises can I do to strengthen my willpower?

I've realized that I'm very mentally weak and it's impacting my success.
I suspect I have ADHD and whenever I get an urge to distract myself, I rarely manage to resist it.
I think what I am missing is the residtance to discomfort that eg. allows sports people to carry on going even when their muscles are telling them to stop. Or the thing that allows people to defy themselves and step into an ice-cold shower.

Unfortunately I am not a person who enjoys sports and a cold shower is only something that makes sense once a day. Can you think of any exercises that I can do here and now in my room, and practice routinely that will strengthen my willpower so that I can better resist my urges in the future?