Do you think you're maybe tensing up when you get this 'fizziness'?
We do need a video really. Are you gripping the horse with your legs at all - this can be anywhere from the seat downwards
What are your heels doing when you ride?
A lot of horses can't cope with a rider that has their heels turned inwards or one who's heels jiggle around
You need to address this problem before you can work on collection because all you have is a horse that's fractious and fighting your hands instead of being soft on them