pluck up one's courage
Also, screw up one's courage. Force oneself to overcome fear or timidity, as in He was really afraid of slipping on the ice, but he plucked up his courage and ventured down the driveway, or I screwed up my courage and dove off the high board. The first term uses pluck in the sense of "make a forcible effort"; Shakespeare put it as "Pluck up thy spirits" (The Taming of the Shrew, 4:3). The variant derives from the use of screw to mean "force or strain by means of a screw."