the.com/bitter end

means The absolute limit of endurance or a situation — the final point where no more effort, hope, or rope is left to give.

from From sailing ship rigging: the 'bitts' were the sturdy posts fixed to a ship's deck that anchor cables were wound around, and the 'bitter end' of a rope or chain was simply the end fastened to the bitts — the part you reach when the whole length has been paid out and there's nothing left to release. Sailors used the phrase literally long before it drifted ashore to mean the last, grueling limit of anything. It's a satisfying coincidence that 'bitter' also means harsh or painful, which helped the phrase take on its emotional punch — but that resemblance is accidental, not the source.