Do you keep your weight in your heels? Perhaps you are pinching with your knees, and when you land there is nowhere for your weight to go. What I would do is practice a lot in your two point position, making sure you keep your knees soft and weight deep in your heels (don't force your heels down, but allow them to sink down with your weight). Something to be aware of is if you have equal weight in both seatbones. I tend to put more weight to the outside, and because I know this, I am aware while I ride and be constantly checking and fixing it.
I know you said that you grab mane when you jump, but if your gloves are slippery, then that may be why you are losing it. Try riding either without gloves, or borrow a pair with more grip from a friend.
It you come back too early, I would practice either a single cross-rail or a gymnastic (depending on your level) and work over that. Can you trust your horse to keep a consistent pace over jumps? If so, I would try this method. Grab mane a little less than half-way up the neck before you start, get up in your two-point, and trot towards the fence. Don't change anything as you near the jump, and make sure you don't lean forward as you take off. Maintain the same position before, over, and at least two strides after the fence, as it helps to keep yourself over the tack and from sitting down early. Keep all your joints soft, because if your knees are tight, when you land it can jar your position and cause you to be unbalanced.
Good luck, and I hope this helps! :)