sounds like you are too tense like others have said.. your trying too hard to get it right. Try not to think about it. Sit up straight with your chest out and your butt down in the seat, hands down and just breath.
Have you tried cantering with out stirrups? It will help you sit deeper in the seat and help keep you from bouncing, but it is scary to think about if you've never tried it. What I did was work at a walk and trot with out stirrups and make sure i kept my legs correct so that I could catch the stirrup if needed really easily and quickly. Then after working that way when I felt confident I'd ask for a canter, if I felt off balanced getting to the canter I'd slow to a walk and try again until I felt right.
Also you can lengthen your stirrups for the canter to help sit deeper in the saddle if going with out stirrups doesn't feel right, then you can move the stirrups back up to your correct position when you feel like you've got the canter down pat.
I bet your thinking back to that 'flying/floating' feeling and just frustrated that you can't get there. I remember the first time I got it and it fills you full of this wonderful energy, It is such a cool feeling and it is frustrating once you've felt it and you just can't get back to it. It will come back, you'll get it!!!