> I guess I have had "burnout" so long, that I cannot anymore feel it.

I've had every possible type of rejection: from banks, financiers, partners, accelerators and developers, over three years of 'booting up startup'. I joke that if I ever have an office one day, I'll print them all out and use them as wallpaper.

And after all I'm still at the very beginning.

I can give only one piece of advice: cold water. 7-10 minutes in the winter sea (<10°C) makes me feel normal. In Finland, probably 3 minutes will be fine. It may sound too obvious for being in survival mode, but any regular physical exercise somehow helps to works under startup pressure.