I’ve been there many times before.
I find the the main reason people quit their games or fall into this mid production depression is because their projects are too ambitious.
A simple way to avoid this feeling is to keep your projects as simple as possible and actually finish them. The simpler the project the better. I am talking about pong simple. Flappy Bird simple. Start with the most simple game you can come up with and finish it, then work your way up from there.
Big games take years to make, and a whole lot of dedication to finish it - to work through the low motivation phases.
You basically need to build up that skill. The skill to finish a game.
There is only one feeling that is better than daydreaming about a game you want to make - and that is actually finishing and playing this very game. And again, that is not going to happen if you dream about making GTA 5 or Call Of Duty.