I've been teaching guitar for 20+ years and here's what I think: to become a truly great player, motivation is simply not enough because it comes and goes. This is too damn hard for motivation to get you through. You have to be obsessed.
Try this. Break whatever it is you are trying to do into the smallest possible challenges. Don't try to play the whole chord progression... get this ONE CHORD CHANGE down perfectly. You have it absolutely mastered. It won't take you very long, and neither will the next challenge if you make it small enough. But every time you succeed at one of these tiny challenges your brain dumps endorphins into your bloodstream. Eventually you literally become addicted to practicing!
This, BTW, is EXACTLY why video games are so damn addictive. The constant reward system.