I may not be the best to help you with this, so you may want a second opinion, but here's what I think....
Maybe hold your horse a bit longer? Do you know your strides to the fence? Hold her back a bit longer and make sure you're keeping track of how many strides to the fence. What I find helps with me is to pick out a spot before the jump to let go of your horse, and do it.
As for your balance, I would say for this one to be sure you aren't throwing yourself too far forward andkeep your heals down and your leg secure. If all else fails, go to a cross pole and practice position over those.
This is what I do, anyway, I just have a hard time putting it into words.