Motivation, willpower, discipline, or anything else alone won't get you through very harsh and painful times. There needs to be no other choice.

I have insane motivation and willpower. I mean 21 days not eating anything while lifting weights and walking 20k steps daily insane. I mean hiking for 5 days without a stop in extreme mountains without food and equipment and with 20 kg backpack full of video gear insane. But when unbearable pain comes, if I have options to quit or stop the pain, I quit.

I've done many prolonged water fasts but finished only half of them as planned. I have started many extreme endeavors, and only half of them were finished successfully. What was the difference? When I had no other choice, I was forced to finish it successfully.

This is why, in many cases, if you are free to do whatever you want, you will quit. To achieve unbelievable results, organize the activity in a way that you are absolutely obligated to finish it successfully. Remove all other options.