I agree with what everyone else said. Like beeniesmom said, take this and use it to apply yourself even more for the next test. Rise to the challenge and be tougher on yourself (not that you weren't for this past test). Do you have friends in the class that you can have study dates with?
It's actually good to be hard on yourself for an 80%. Shoot I'd be thrilled if I got an 80%. But I think you know you can do better and that's what you need to work toward. If you feel comfortable talking to the teacher like Karen said then go for it.

You can do it!!