Hi, like others say I def use a light seat when cantering out on hacks, it can feel much nicer and more 'with' the horse esp on a big mover, but I do still sit up for the upward and downward transitions as those are the bounciest parts and also have my eyes open for any hazards, anything that might make the horse spook or veer around or if s/he's getting a bit excited and at risk of throwing in a little buck or picking up too much speed, if so I immediately return to my much more 'defensive' upright seat with centre of gravity further back and also make sure I have a good hold of the horse's head and can keep that correctly up and on the vertical, looking where they are going, not down and looking between their knees. It's nice to let the horse stretch out a little, lengthen their frame and raise their back during a long canter, again the movement will usually feel smoother that way and more comfortable for them but you are more vulnerable to loss of control or balance particularly if you've allowed your reins to get long and the horse to fall onto the forehand a little which is easily done as a novice.
If picking up too much speed my order of things to do is ( first shorten reins up if too long), little gentle half halt first, just as a reminder, if that doesn't work return to upright seat position and stronger half halt, maybe a soft 'wooah now' voice aid, if that doesn't work big half halt, use both reins, body weight well back and down in the saddle, loud wooaah command (serves also to tell my hacking companion I'm being run away with so they can stop too which is as helpful as anything else usually 
, also exhaling the word woaah in a soothing tone reminds you to breathe and not panic!), repeat until they slow down (remember when half halting, even when they're not responding, you do need to take and then let go, if you just take and take some more, hanging hard onto the reins in a panic they'll likely just pull against you more and not stop, so do a big take if need be but then an equally big release). If truly, urgently, out of control turn the horse's head and body using one rein to spin them in a circle, using a handy hedge or bush as a barrier if one is available (the one rein stop technique, useful to know and something to ask your teacher to show you although it's quite harsh on the horse's mouth and only to be used in genuine emergencies so not something you'd practice in earnest - I've only ever had to deploy it a few times in my life but it's worked both times)
So yes like PPs say ask your instructor if you can practice light seat (sometimes they'll refer to it as 'jump position') during your lessons, on the lunge if need be, you may find it hard work at first as requires quite a lot of leg and core strength to hold it correctly and not just flop forwards, I practice by doing one lap (or one circle if it's too hard at first) in 'normal' rising trot or seated canter, one lap 'hovering' above the saddle but standing up straight (advanced step, put your hands on your head if on the lunge to prove you aren't balancing on them), then one lap in light seat (arms out to the side like an airoplane if on the lunge). Think of the movement between the hover and the light seat as taking your bum and bodyweight backwards and over your legs, rather than folding forwards and carrying your weight over the horse's neck. You should at any time be able to snap quickly between any of the three positions without losing balance or relying on your hands to hold on. And like I say, don't forget to pay attention to your contact and where/how the horse is carrying himself as well as this will def affect your level of control...
Finally don't worry too much, it will come, feeling bounced all over the place is really common when you start out especially on a bigger moving horse than you are used to, any tension or nerves will make it worse (one of the hardest but most useful skills you can learn as a rider is how to relax your muscles when you are feeling anxious rather than stiffening up), anything you can do to improve your fitness and core strength will help, otherwise just saddle time and experience and you'll get there. There's no shame in just doing short bursts of canter and then stopping either, as a novice the people you hack with should be able to accommodate and TBH if you are on a nice big long striding horse he can probably keep up at a smart trot with a slow canter from a smaller horse anyway so if you need to trot for spells then fine!