Like others have said: balance. Verticals for the rider seem less intimidating, but for horses, they are much harder to judge.
along with what everyone else has said regarding strengthening her hind end… A great exercises for this is to trot/canter/balance/trot/canter/balance/ to ask for that downward transition to be “up” take a couple of trot steps until balanced and then re ask for the canter. This is engaging that outside hind, while also building balance. Also, LARGE cross rails (think, the center of the cross rail will still be low but taking poles to the top of the standards to create a narrow “low” area. I wouldn’t, at this point in your horses training, hike up the poles all the way to the top, but certainly make a larger cross rail with a trot pole and ask to trot in, straight (almost a “forced” straight) which will REALLY build the hind end strength. That along with low cavalettis at the trot and grid work with a a cross rail, vertical, oxer (maybe say, cross rail, three/four strides, vertical , four strides, oxer… the horse will see the oxer and sit back in preparation and perhaps respect that vertical more)
Anyways, idea. I know your horse does oxers “well” but really struggles with the difficulty of an upright fence. Can you do the cross rail exercise I just mentioned (raising the poles to make a larger X) withOUT the spread, and have the vertical portion of your “oxer” be the same height as the low point of the cross rail? No spread, cross rail behind vertical.
Also, at this point, I would not be cantering fences if there is rushing and unbalance with more than likely a weak hind end. Lots of trotting into things, trot poles can be helpful too.
If your horse is knocking the pole down with the front, it’s almost guaranteed they’re not set up right and lack the strength. A horse who catches behind may be caused by a rider sitting up too fast, too, just like a horse knocking in the front, caused by a rider’s lack of balance and perhaps too ahead of the motion, with not enough balance and the horse “balancing” purely off the hand. When you become airborne, if the horse is simply relying on your hands for support, unless you’ve got magical powers that can hold a horse mid air (which, ok, magical maybe not, but for the less experienced rider and/or horse, near impossible) the lack of contact over the fence is going to cause some problems.
So, lots of trot work. Lots of basics. Don’t put the fences up purely in hope the horse respects them more. I know for me, riding under several GP riders, we work mostly over small fences (2’9" and under, a lot of time, smaller) but balanced, correct, straight. You don’t need height to gain the respect of the jump, all that will end up do is causing a loss of confidence in both of you.