@JohnStrong thank you so much for putting these two here and also asking the question. The Balboa speech is a classic, it's about resilience and grit (two very different things) more than anything else. I sort of skimmed the Mind Pump video and it is OK-ish but ... there is also a certain amount of muddled thinking in it not least discounting motivation as a feeling which therefore should not be examined.
Neuroscience has given us, over the last five-ten years, a very good idea of why we do some things or fail to do them. We know, for instance, that motivation is a specific emotion that arises when the discomfort we feel in one particular physical and mental state reaches a threshold level that is, to some extent, unique to each individual. We are then moved, virtually against our own conscious volition, to take action. For instance, a person who has had a life-threatening health scare is highly motivated to stay healthy and watches their nutrition and never misses their training. Their motivation over time will change, particularly as they get fitter and feel healthier but, at that point, sustainability is maintained by other neurochemical processes involving serotonin and dopamine which affect mood and are linked to the brain's reward system. Their entire identity and behavior will be so linked to training and being fit and healthy that not doing what they have to do will result in them feeling disgruntled and 'off' which, in turn, will lead to that internal sense of discomfort that brings about the motivation we feel to do things.
So, to get to your question of "How do you get back on your horse?" - to begin with you need to have an understanding of why you 'fell off it'. What made you stop? What keeps you from maintaining a fitness routine day after day? When you identify the obstacles and remove them, the task itself becomes easier. When you have identified your reasons for wanting to be fit and healthy and clearly felt their imperative, then your motivation to do so is back.
This video, short as it is, helps a little:
The thing to remember is that 'discipline' can't be consistently hard. That way we fail. So what we do has to be made as easy as possible to do. Then the excuses for not doing it become harder to swallow.
I am not sure how much all this helps and it probably raises quite a few questions so don't hesitate to ask me anything you feel needs greater clarification.