Starting a new diet is the worst thing anyone can do. Does anyone start a processed, junk food diet? No!
When choosing to change your diet to a more healthier version, should be considered just that, a change in diet. Along with any change in life there is always a transition period. During this time there will be testing and easy times, just as there are in everyday of life, the transition allows you to kick old habits and learn new ones.
when poor decisions are made, don't consider it 'falling off' or 'failing' think of them as opportunities to learn why you made the choices you made and how can you prevent yourself making those same mistakes again.
If, when you make a poor choice and then go on to use it to make poor food choices for the rest of the day/week/month, then in reality you are not ready to change or committed enough to yourself to make a positive impact.
If this is the case, wait until you are ready, when you really want to change, otherwise you will keep considering yourself as a failure and relate dieting as a bad experience.
As far as motivation goes, when you really want something and have put in place the right mindset, your change in body, weight and energy will offer you all the motivation you need. It's all in the mind.