Don't rely on motivation for anything. It is fleeting and unreliable. Discipline, however, is unyielding. Force yourself to follow through. Things are going to come up in life that get in the way of your goals but that's no reason to let them stop you completely.

Can't do a whole run? Do half of one. Don't have time to go to the gym to lift? Do pushups. Any action is better than inaction. Don't like the food you need to eat to meet your goals? Tough. A little discomfort is worth the change, and at the end of it you'll have a mind as tough as your body.