I don't do gyms. I prefer hikes and vigorous walks on my property. I live in the mountains so they can be pretty strenuous. I do these because I like being outside, but that's as far I look for motivation. Maybe you'd rather do something other than the gym, but after that it's all in doing the activity. Somedays I don't feel like doing it and I do it anyway. Motivation is a lie. It's a myth. If you look for motivation you'll never do anything. Regarding discipline, you don't "bring back" discipline and you don't find it. You develop it by doing the same thing over and over no matter how you feel. That's literally what discipline is.