Everybody probably gets a drop in motivation when the going gets tough. That's where discipline comes in. Discipline isn't some magical pill that makes everything better.

It's the choice you make when you're mentally uncomfortable, do you keep going or do you retreat? Discipline is to keep going until you physically can't anymore (for example, I'm a track athlete, and we have these lactic acid trainings, after a couple 200s at 90% top speed, your legs are burning, but you force yourself to keep going, because it's all in your head. That's discipline. Then sometimes your muscles cramp up, and you're forced to stop (that's when you physically can't go anymore, unless you want to injure yourself).

Track helps me a lot with this even in other aspects of life. Sometimes I'm in front of a hard problem, and I can't even understand half the terminology, much less all the concepts. It's easy to get discouraged, but I tell myself "try to understand one concept at a time" just like "one set at a time in track". 6 hours later, I've "mastered" the problem and can tell you the ins and outs of everything related to it, but not without having to reread wikipedia pages 3 times and open 30 tabs from google searches.