Do you ever just . . . feel like you're the time remnant? Like, putting subjective relativity aside for a moment and assuming there's some other timeline out there where everything in your life has happened quote-unquote the right way - do you ever feel like you were supposed to be that guy but at the end of the day some other version of you got to be?
Cisco gets chills over this on a regular basis. This is mostly because he knows that time remnants are A Thing. He's met a couple. Seen most of them get brutally wiped from existence, too, which is a pretty lame consolation prize. Then of course there's the whole interdimensional crystal ball his brain's become, which means he occasionally plays fly on the wall to the zany goings on of his dopplegangers.
So, yeah, sometimes Cisco freaks out over this very specific life he experiences in real time. Out of an infinite number of universes, each with their own infinite number of timelines, he wound up stuck with this reality, this existence, and will have to see it through to the end.
From a subjective, relative viewpoint, he's got no beef with the status quo. This is the normal way of it. Everybody gets to be born, spend some years doing some stuff, and then they die. Point A to Point B. Tidy. Cisco's a scientist so he is more than down with trusting the observable facts of his physical realm, but also his best friend is a superhero who is known to run fast enough to punch holes in the fabric of space-time so where exactly does that leave the scientific method.
(Schroedinger's Timeline: reality defined by observation. Peeping into other realities, bringing future knowledge to the past, how can they be sure they're not glitching out the multiverse every other Tuesday?)
It's not that Cisco is displeased with the years he himself has spent doing stuff. Actually, compared to some of the Earth-N Ciscos he's scoped, he's not doing too bad for himself. Not every world is fleshed out with Barry Allens and S.T.A.R. Labs and Big Belly Burgers. Not every Cisco has as strong a sense of self, or knows the thrill of saving the world multiple times over.
What plagues him, it's more like . . . the sense that . . . regardless of the course this life has taken, maybe none of it matters in the end? This doubt that flares up from time to time and nags at him: maybe, at some point in the past, he turned right instead of left and walked straight out of the Alpha Timeline and into some dead-end what-if pocket universe without even recognizing the error.
It sucks, too, because this can get really dark when he brings Team Flash into the equation and imposes his existential crisis onto these versions of themselves. The ones who are stuck here right along with him. What would Barry think, being told that he might not be living the life he should have had? Baaaad example - what about Caitlyn, though? What about Joe, or Iris?
What if he sat them down and told everyone he cared about that every second, every hour, every day was another one down the drain, meaningless and empty. Doesn't matter what the difference between this life and that life might be - what if there wasn't some better life out there that they weren't living, just the right one?
He can't, of course, because with his capabilities they just might be liable to believe him. So he suffers alone with his neurosis, wondering where he went wrong; wondering if, for example, the guy who wore the Nyan Cat shirt instead of the Turtles shirt on the third Thursday of last June went on to feel as pointless as he does now.