Get back extension?

a few years back I dislocaited, and the tip of my humerous fractured, i was in a cast for a few months.. and went to physical therapy, they just told me to do really light weights blah blah blah with that arm... so i didnt really listen because i wanted to get back into playing sports.. but now my arm is a lot weaker; and it is my dominant arm.. i've continued to go to the gym and play sports with friends sometimes.. but it begins to ache.. and doesnt work as well as it used to, and i am young.. so i want to not have this for all my life.. and suggestions please.