Tell me if this rings a bell.
You watched a really intense movie about a character that overcame the impossible. They struggled, they fought, things got worse, they didn't give up, and then they won. Maybe it was "The pursuit of Happiness" or "Miracle". You feel so inspired when you're done and you think, "I want to be like that. I want to change my life"
Or consider another possible moment. You've been eyeing everyone on social media and you find a fitness account or two that AMAZE you! They are in incredible shape, they show dedication all the time (which is falsely making you think they have that "motivation" 24/7) and you think "I want to be like them. I want their body/life style." And you set out to do it with the feeling of "I can do it! I want to do it! I am willing to do it!"
ALL of that is true! What isn't true is that that feeling carries with you everyday for the rest of your life (or even for a week/months). Motivation is not meant to be something you have all the time, simply because that motivation is caused from chemicals in your brain; specifically adrenaline and dopamine. You CANNOT have those coursing all the time every day. So step 1 is: forget motivation. It no longer plays a main or key role in your actions and dreams. You must decide to "do it" for something else (I'll get there in a moment).
Heres a great analogy. A forest fire does not start and continue to burn because the tallest tree bursts into flames. A forest fire continues because of small pieces of brush catch fire throughout the forest. The fire can start for something as little as a cigarette butt. A cigarette 1/2 an inch long can cause a fire to be thousands of acres over time. Stop trying to burn your forest at once.
Every day, it is a small step, as long as you keep that 1 bush burning, you can relight your forest fire. You're one brush fire is your "why". Its that one reason you come back to every time. Maybe it is your kids. You want them to grow up watching you push yourself and teaching them to be strong and courageous. Maybe your why is just an internal drive you can't explain. A gnawing feeling that you must always push on and push yourself to be better every day. You never want to settle. Maybe your why is your don't want to have regrets that you didn't try or "what if" I had just done it and STUCK WITH IT. Maybe your why is because you LITERALLY cannot feed your family or get a decent job without doing it. Your why is going to be personal, but we all should sit down and write it out.
Motivation may be a good place to start for a day or two. but after that you just have to DECIDE. Burn every bridge going back to the way you were before, there is no going back. Don't let it even be an option. Throw all the crap food out of your house, set a permanent alarm for 5am, meal prep lunch every single week, only choose 1 night a week to go out, no exceptions, to stay late at the office to get more work done. What ever it is, its the new you. Its not a fad. Its complete character change.
When you get tired, its easy to start skipping the small things. Its like when you're running a 10 mile race and you get 5 miles of the way through, why would you turn around and run back? You still have to run 5 miles back, or you finish and run 5 miles forward. The time will pass either way, you may as well keep going.
The biggest problem is that we give ourselves the OPPORTUNITY to fall back to old habits, we don't cut them out, because we don't know what it will be like without them. The unknown is terrifying. But as long as you have your 1 burning bush, and you keep your list of whys, that is all you need. Motivation is fleeting, but you just have to keep doing the small things. That is called dedication.