be at the end of your tether
be at the end of (one's) tether
To be completely worn out, exasperated, or exhausted; to have no more patience, endurance, or energy left. The baby's been crying all morning, and I haven't slept properly in days! I'm just at the end of my tether! The bosses are at the end of their tethers with the way this project is moving, so they've decided to bring in some outside help.