I find I have an easier time taking things in parts. I conceptualize what I want and just write everything down. then I go through what I wrote and think of how it can be done practically and assign each thing I want to do as a task. The real trick comes here, the tasks don't need to be done sequentially. Yeah some stuff requires other stuff, but whenever I look back over my notes I find a can make skeleton parts for different sections and just finish them off when I'm doing the other required thing.
The reason this has been a good trick for me is I don't feel forced into doing anything. I can work on whatever part I feel like at the time. When I get into a work groove I switch to the more grinding stuff that needs to be done. It massively helps remove the boring stressed bits.
The 2nd thing I do is just find something fun in the game that's already done, like 1 battle in an rpg, or a jump sequence in a platformer, and just stop and do that whenever I feel fatigued. It reminds me how awesome what I am doing is, gives me a break, and gives me some play test perspective on what the game should feel like throughout.