It's that "I can't believe THIS is supposed to be life-affirming"
ennui that makes you regret your tired existence.
Characterized by a deep sigh of dissatisfaction, it's that miserable feeling you get post-coitus where you realize you spent countless hours convincing yourself and some other person that this was going to turn out well.
"What's that feeling you get after sex?"
"What, you mean ennui?"
"Yeah that's the one."
"As she started to snore contentedly, I lay there confronted with a gnawing ennui, a confounding disappointment that crept up into my brain as I realized I felt nothing but contempt for the person beside me."