I came to the insight that if I violate my trading plan then I do something that is not good for me in the long run. Why then should I do it? Of course, a well developed method to trade and a well thought out trading plan are necessary prerequisites. In addition, the person applying such should also be convinced that the method and plan are really good, an attractive business idea. Executing then can become quite easy. But only then. If there are nagging doubts, uncertainties and unanswered questions, unresearched but relevant areas, wishful assumptions without evidence etc. then method and plan are probably not yet ready.
design and understand you own trading methods. If you are not disciplined it means that you do not understand your trading methods. "The heart has its own reason that the reason does not have"--your heart gets you in trouble when your reason does not understand everything it should.
When I feel undisciplined I go around trying to feel more undisciplined. I observe where the feelings lead.