at (one's) wits' end
In a state of distress because one has no more patience or mental stamina, often after having dealt with some kind of problem or difficult situation. The baby's been crying for hours, and I'm at my wits' end! I'm already at my wits' end, so if this train gets delayed too, I'm going to scream.
at one's wit's end
Fig. at the limits of one's mental resources. I'm at my wit's end with this problem. I cannot figure it out. Tom could do no more. He was at his wit's end.