The only thing that keeps me pushing hard is the fact that I'm doing a game that I really would like to play and where I can basically put all the feautures I want in it, whenever I want.
If I had to program crappy flappy bird clones, well then that is _work_, and surely I would need some motivation sooner or later (eg money).
But as soon as it's not work anymore, then the only reason why I would do something is because I like doing so. I like creating rpg that I would play, and I like the possiblity to shape the game the way I want even more (you know, crafting, alchemy, monsters, graphic style...)
You should start questioning yourself about why "now and then is so hard to stay motivated", because if you're doing the game that you really want to do, then it's pretty easy to stay motivated.
That doesn't mean there won't be any up and down, to be clear. But it will be much easier to stay motivated and go ahead if you do something you really want to see coming to life.
ps: of course you have to understand what are your physical limitations, and work around them: you won't ever be able to reproduce the graphics quality of skyrim, but adopting evocative and simple art styles you can surely work around that.
Leonardo