For me personally, I've learnt that I should build only what my hands can hold. I have to know clearly if I can handle something. If there is any uncertainty about whether I can do something, then chances are that I lack experience, and that inexperience will lead me to certain failure unless I can gain the necessary experience.
What I try to do is create boundaries/limits to my work, so that the work doesn't exceed what I am able to handle. If I spend too much time imagining things, I overlook the boundaries to my physical limits, which make things overwhelming. I also think that focusing on the essential parts are more important in the beginning- things that hold the game together, or run deep throughout the game, like a dialog system or inventory system, menu system, save/load system, etc. Those sort of things should be created in the beginning rather than later because much of the game will rely on those. My specialty is with art, but I don't focus on art in the beginning at all, unless it is necessary to visualize something.