Zeitenschmerz

The distinct melancholy or mourning one may experience when thinking about the potential pasts, presents, and futures one may have experienced had they made different decisions in their life; a negative reaction to the endless stream of possibilities and 'what if's one accumulates throughout their lifetime

"Dude, I've been thinking about how my life could be right now if I did get together with that one girl from school, and now I'm super bummed."
"That's just Zeitenschmerz for you."