I just want to say I don't think it's really a smart idea to have a rider that is learning to jump to teach a green horse to jump. That is just my opinion..I think it's best to learn on a horse that knows what he is doing, then pass on what you learned to your green horse.
But anyway, when I was learning to jump, my trainer would always tell me; wait for the jump to come to you, try and sit all the way until the horses front feet leave the ground. This is a little difficult to do if you don't have strong legs and good feel for the horse. Going into 2-point too early can make your horse take a too long of a spot. Since you are only trotting that should not be an issue, unless the horse takes a few canter strides before the jump.
You should try setting up some ground poles, at a trotting distance (about 4.5 feet apart) then after about 3 or 4 ground poles set up your small cross rail (also about 4.5 after the last ground pole). Then just trot right through the whole thing, even the cross rail...don't two point over it, just post over it. This will help with seeing the correct spot, and help you get a feel for your horse.
The reason she could be switching her lead over the jump is because you are throwing her off balance.
Hope this helps.