Here are a few options… “Quit whining.” (US English, extremely direct/possibly rude) “Complaining isn’t going to help.” (Direct and firm) “Stay positive!” “Let’s keep a positive attitude (about things/this).” (Clear and direct but gentler) “Surely there’s a silver lining.” (Gently suggestive/very mild)

Stephen's answer is fantastic, I'll just add my favourite (very rude) response to complaining: "Cry me a river!"

The other answers are good. I will add "Quit grumbling!" "Hey, Relax!" "It's not a big deal!"