sometimes you can't rely on that desire to work staying for as long as you need it to
the best thing is to just do it, and somehow hold yourself accountable. maybe get other people in on it that'll help make sure work is getting done (and they can also help move things along more quickly and work on things you might not enjoy as much as them)
you can get a calendar or something and mark off every day that you work, and don't break the chain. that's something a lot of people do to keep their personal work consistent and efficient.
also, if it's something you'll ultimately be able to play with, it really helps if you can somehow get at least a prototype working. for one, it helps you know if your idea is actually going to be fun and functional, and for another, it means that all your work is going to be building off of an already-playable platform, and you can test things right away, and have fun doing it. this also means that you'll have a better idea of what the project could use to make it more interesting through actual experience with it, rather than just theory.