Have you put so many hours into practicing and enjoying numbersense? Do you wish it could continue past high school? Perhaps, attending backgammon tournaments is a satisfactory alternative that one can continue indefinitely. Backgammon isn't numbersense, but it often involves quick calculation, and I believe that people that like numbersense tend to like backgammon. It is said to be the most "lifelike" game of chance: the closest to a Brownian motion. Therefore, I find it more philosophically intriguing. Also, unlike chess, a master can still have fun playing a tyro, because tyros still beat masters. Weather it be the challenge of increasing your winnings from 50% to 55% against an equal or from 75% to 80% against a newbie, it's always present.
To learn, I suppose there are lots of videos on youtube, etc. It's not hard. Here's one from poker expert, Gus Hansen
http://www.youtube.com/watch?v=XwT3o1lZX7E
Cluster counting, which is an area where most players fall short, is an arithmetic intensive skill,
http://www.bkgm.com/articles/McCool/cluster.html
Finally, another important skill in the game is to be able to "count shots" and figure out how many possible dice rolls will end up poorly/goodly. If you watch PJ Simborg's videos, you will certainly learn about that.
Best of luck,
Aaron