echo what madam max said but also transitions within a pace........slow the trot down to an almost trot on the spot then push it on again, keeping a contact though otherwise they will fall on the forehand
Direct transitions to help your horse use its bum more - also schooling over alternate raised trot poles may help (e.g. left side raised, then right etc)
Trying using a jump/light seat along with your leg aids, that way your body is out of the way of your horse and it should enable her to go forward more easily, once you get that, you can pace her by doing alternately a few steps of rising trot and then a few of jump seat. Also trying doing inside and then outside bend every few paces on a large circle to help her relax and lower