Motivation is fleeting -- if you're unable to sustain it, you're no longer efficient. This applies to working out, your job, your school work, and even your hobbies.
When you're no longer hyped and madly-in-love with your work, motivation isn't going to help you get the work done. Discipline is what will help you achieve your goal. Even athletes get burned out, and they have days that they just rather snooze in bed -- but there's a goal, there's something that needs to get done. It's discipline and the desire to get a job done, that keeps them going.
When I was Powerlifting...
I had to change my lifestyle. I had to be disciplined to my sport.
If I wanted to drink before my powerlifting meet, I couldn't.
If I was trying to cut weight, I put down the fries and burgers.
When my coach suggested sprints -- I shut my whiney mouth and did it.
It wasn't about what I felt like doing, it's what I NEEDED to do.
It was discipline that kept me going.