HUMONGOUS oxer

I had a lesson with Meg tonight and John had us jump the same fence while he gradually raised it until it was 3'11", and then a 3'11" oxer.  It was freaking gigantic.
It was also frustrating, because both directions I couldn't get the distances right consistently.  John showed me where to turn, and then they were smooth as silk, but how could I not think to adjust something like that when I keep getting odd placements over and over?  John showed me how I would ride deep into the corner to try to rev Charlie up because the fence was big, and it would end up a half stride off from the fence.
Charlie jumped them beautifully, even adding a stride a few times and really bending up and over, but my position was horrible.
Although they were huge (intermediate height) I'm cranky because I wanted to be magically good at the big ones so that I could level up, and all the goodness came from Charlie.  I just made his job harder.