Whenever I think I could never do something, I try to break it down into doable pieces. After falling a couple of years ago, and limping for close to six weeks, I thought I'd never be able to walk effortlessly for 90 minutes ever again. Baby steps. And though I still feel occasional foot pain if I walk much more than an hour at once, I believe that eventually, those 90 minute walks will become a daily thing again. (Because after all, two months after I fell, even 10 minute walks hurt.)
Same with weight or IF. Don't worry about losing 100 pounds. Lose 10 pounds. Then do it nine more times. Don't worry about going 24-48 hours without food. Start by skipping the meal that is more about habit than hunger. Then go from there. We're so habituated by certain things that we mentally constrain ourselves to sometimes fail before we even start.