Help my badminton partner it super competitive!!!!!!!!!?

Okay so I made my school's badminton team and my teacher picked our partners for us and my partner is really competitive! We play games when we practise and whenever the other team gets the point or when we start losing or when she misses the birdie, she gets really pissed off or sad and starts playing really bad! I tried saying things like your really good at badminton and nice try (when she misses the birdie) but it doesn't help! We have a tournament tomorow and I don't want her to get mad when we lose! What can I do???????