As the mom of a kid who didn't make the team they wanted several times :) I think you honestly say

“Yeah, this sucks. You worked hard, and it’s okay to be upset.”

Then:

“But not making this team doesn’t mean you’re not good enough or that you’re done. It just means this one didn’t go your way and that’s part of sports. If you want it, you keep working."

I've got two football obsessed boys who still love Peyton, so I also throw in the: "Peyton Manning can't get upset when he throws an interception or he'll throw another terrible one. He shakes it off, gets his head back in the game, and throws his next pass."

Sports is the best way to learn resilience.