Basically: I make blunders. The most disgusting, reprehensible, indefensible mistakes that I would've discarded had even one neuron in my brain noticed it before I moved. I know everyone experiences them to a greater frequency than they would prefer. But I can say that they are without a doubt the number one deficiency in my play, and also the number one thing detracting from my enjoyment of the game.
I play mostly online, and am around 1900-2000 on lichess after several on-and-off years of chess. Up until recently I have always played with long time controls (~30 minutes with increment), although I have started experimenting with blitz in an attempt to remedy my tendency to take an unreasonably long time studying certain positions and still making outright blunders; this hasn't helped. Regardless of the time controls, I still always feel myself at an imbalance compared to my opponent because while I may have slightly superior technique that'll allow me to chisel fractions of a pawn of an advantage each move, I am more prone to naked blunders throwing the whole game away in one move (and unless it's blitz, I have little hope of recovering since my opponent can play consistently).
I really don't think this is just in my head. I've tried "slowing down" (even though my calculation/processing style is already very laid back), I've tried "checklisting" for blunders (but the checklisting just becomes a hollow routine that can't even cover all the dynamic ways I can screw up), I've tried just waiting throughout months of playing but I don't feel I'm adapting. I love studying and practicing the more refined aspects of chess, and I feel like I'm capable of it the vast majority of the time, but I'm really only as strong as the weakest link in my performance. Has anybody else that's felt the same as me managed to improve their recurrent blindness?