if you feel it - run away from PC. I mean it, grab a smoke, drink tea, strech, watch porn, meditate, play Zelda. Give your self 15-30 min to chill. If you just lost good ammount of $ walk away.
If you in the middle of the level and cant do it - deep breathing, and just clear you mind, brath and no thinking about last hand, when break starts do what i described before... not at once, pick few.