The thing that sucks about going 100% on something difficult isn't the thing being difficult, it's when you fail relentless for an extended period of time that makes you burn out.
My feeling when being in startup mode was essentially the sunk cost fallacy. I've already put this much work in, success must be around the corner. But I look at some of my peers with similar mentalities and you can't help but feel they're too focused on the day to day without realizing the product and demand is garbage. Life is short, enjoy it.