Everyone says the same thing to you after a bad show. Don't worry about it, it was just one show and everyone has a bad show every once in awhile. And this might be true, but it never makes it feel any better. Nothing stops you from thinking about it on your way home or later that night as you lay in bed.
One of the worst improv scenes that I've ever performed in front of an audience has to be a "Butt Prov" scene I did. I initiated the scene by standing on a chair and bending over while holding my butt (comedy gold, right). My teammate joined me on stage and mirrored my actions, a classic improv move. This moment was caught on camera and is pictured below.
Mirroring my actions did get a slight laugh, but for the next 2 minutes the scene was completely laugh free. We made no strong character choices, we didn't name each other, we had no point of view, and we didn't even look at each other. It didn't even work as a wacky, stupid scene. We just stood on those chairs holding our butts. The scene ended with our teammates trying to save us by acting like they were being shot out of our large intestines. I never typed that sentence out before, but by describing it right now I realize how horrible it was and I would like to apologize to everyone who witnessed it.
I sucked that night and I'll suck again. That's the beast inside the beauty of improv. No one is funny all the time and you can't expect to be constantly great while making stuff up on the spot. The best you can hope for is becoming so good that you only suck every once in awhile. So how do you get over a bad show or class? From my personal experience the best way to get over a bad show is to have a good show the next time you perform. Learn from what happened, but forget about it and move on. I remember great scenes over bad scenes any day. If that doesn't work you can always do what most improvisors do and consult the bottom of a glass.
ADDITION: One of my favorite improv teachers, Craig Uhlir, would always say, "You can be upset about a show for however long the show was, then realize that you're just playing make believe with your friends."
To end on a positive note. Here is a video clip from a recent show.