Spiralling into self-doubt is perhaps the only failure mode I'm most careful to avoid as a founder.
You have days where everything feels like it's failing, the company is going to die, and nothing is worth it. Two days later, everything is back on track, and you're firing on all cylinders.
Every founder goes through this. The only thing that kills you is letting it become a downward spiral.
The goal on those days is just to survive. Go through the motions. Wake up, brush your teeth, go to work, sleep.
The army makes you make your bed in the morning for this exact reason. You get one thing done, and you feel slightly less terrible. Then another. And it compounds.
I've had years of going through negative stretches like that. You build the muscle eventually and learn that the feeling is not the fact.