You don't. You need to have a goal and clear understanding about why you are doing what you are doing. This is the same with pretty much all activities that require significant effort - motivation is a brief blip that eventually withers away once you start struggling. What you need is discipline, planning, and regular routine. Plan (allocate some time each day/week) and do this regularly. Can't take it anymore? Make a coffee, take a walk, rest for a little while, take a nap, whatever, and then try again. Motivation is not something that you should be constantly chasing in the first place.