square (one's) shoulders

To prepare to do or deal with something difficult or unpleasant; to demonstrate courage, resolve, or determination. I know that you're nervous about the game, but you just need to square your shoulders and do your best. Many people in the audience were booing when her name was announced, but she squared her shoulders and stepped onto the stage to perform.

Prepare to face adversity, as in She knew it wouldn't be easy but she squared her shoulders and faced the hostile audience. This expression transfers standing erect with the shoulders pulled back, forming an angle much like a square's right angle, to the situations calling for this stance. It was first recorded in 1819.