I'm genuinely wondering what the hell the problem is with your controllers/systems/hands, because it really isn't that complicated and you don't have to hold any buttons.
As an enemy readies an attack (indicated by a red triangle appearing over their head), Connor can counter their attack by pressing the "O" button.
(obv on 360 "O" = "B")
And even when I *DO* get a successful counter, I just shove them away I don't kill them, which makes the fight last forever...
Right after a successful counter, you have to issue a follow up command. X to kill, A to disarm, B to throw.
It tells you in the tutorial. It tells you in the in-game manual. People have said it in this thread numerous times.