With discipline you will be successful. Motivation will run dry. At this time discipline must take over. Quitting is the only way to ensure you will not succeed. Stay the course and get the reward from all your hard work. Anything worth having is not going to come easy. Hard work I have found always pays off. The worst days in the gym may not change your body, strength, or stamina. Those days however are the best for changing your mindset and getting stronger mentally. Facing adversity and getting mentally tougher is going to make you that much better at facing struggles in every day life.
Discipline is doing stuff you hate to do, and doing it as if you love it!