Focus on discipline more than motivation because it will get you farther in the long run.
I'll use myself as an example. There are days I'm just not motivated to work on my game. I would much rather watch Youtube or find a cool Steam game to play. But because I am disciplined, I get up at a set time every morning and work on my game for a solid hour. Don't get me wrong, the progress can be incredibly slow the way I do it, but progress is still being made. Some mornings, if I don't feel like programming, I work on something else that is more enjoyable, like modeling spaceships in Blender or rendering explosions in Embergen.
You need to be absolutely convinced in your mind that the end result is going to be worth it. Think about how it will make you feel to finish it.
Learning the skills to make games -- that's the easy part. The hard part is the staring contest with your computer monitor to see who gives up first.