Sometimes you just need motivation, but most of the time you need consequences
The first time I did a heavy squat, I nearly failed the rep. I was stuck a quarter up from the bottom for about two seconds. The only thought going through my mind was that if I failed, then the entire weight would fall on me. I finished the rep.
Motivation got me to the bar, but consequences made me finish.
Most of the behavioral changes that I try to implement for myself don't have the benefit of a heavy weight hanging over me if I fail. But the successful ones have something similar - a kind of weight that is attached to my identity. If I fail, I hurt my own sense of identity and that is something I don't allow easily. The unsuccessful ones, on the other hand, had nothing of the sort. I didn't bind them to my sense of self and thus produced no consequences for failure. When I failed, I didn't even notice.