Anything which involves competition I tend to clam up. I can do it but I get very stressed, and then don't always succeed, despite putting in a lot of effort, which is frustrating.
I tend also to view it all as threats, such as if a guy beats me at Mario Kart or Laser Quest when my SO is around, then that's a threat to our relationship-and an even bigger threat if I am trying to 'woo' the girl in question, because who wants to date a loser (or Mario Kart or bigger things?)