I can't canter correctly!!!!?

I can get my lease horse to canter, but I never sit down. I don't know why but I'm always bouncing around. If I bounce forward I freak out halt. I think it might be because I stiffen my knees. How do I get it to stop. If I really try to stop bouncing I end up standing up in 2-point position. Please tell me how to stop it!