Your will is like a muscle, flex it! You must first exercise it and then push before it gets stronger. This necessarily involves short term pain, and most times long term. But be assured that the improvements will come and will touch your character in a most positive way.
That being said, when you are hungry, wait another hour before your meal. When you are pissed, take a deep breath and settle in, everything must not end up in a fight. When you are facing a temporary financial set back, learn to make do with the little that's available and be cheerful; someone somewhere doesn't even have a little to manage. When you are labouring over a difficult task and your mind is prompting you to pick up the latest magazine for a break or to get up and go talk to a friend, curb the impulse etc. Soon you will be able to sit for hours in a precisely concentrated state.
Sir Issac Newton, one of the greatest classical physicists the world has produced, once said: "if I have done the public any service, it is due to patient thought." Newton had a remarkable ability to sit quietly and think without interruption for very long periods of time. If he can develop this so we can!