I agree that it is exhausting. I'm 2 months into a solo founded startup and the mental fatigue is not what I expected. Reflecting on it now, it certainly makes sense, but I just didn't know that it can be so taxing.
I've told myself that if I make it to a next round of funding I need to find a partner stat.
He chose to speak his mind, and I believe his opinion was mean-spirited and unnecessary. Sometimes, employers fire people for that, and that's the way the world works. I don't see it as a problem.