There is no breed called a pitbull - almost all of them are bully types, mixed with something else. Many of those "something else" dogs can be hard to train. That means you need to work harder and longer - there is no special secret. The dog isn't stubborn so much as it is necessary for you to figure out what motivates that dog. And more exercise! A tired dog is less likely to be distracted. In this case, you need help. Contact your vet for the name of a good trainer - someone you found on the internet may or may not be qualified, and may or may not be very good. The trainer will watch you with your dog, and teach YOU how to train the dog. It involves repetition, and effort. Many of the difficult dogs just need another approach and a good trainer will find that for you. You have a large and powerful dog that is going to be a liablity for you if you don't get him properly trained, and I'm sure you know that.