maybe try to make your signals VERY clear. you want to take your hand thats not holding the lunge whip and put it the way you want your horse to go. and use the whip to block off where you dont want him/her to go. hope this helped
Besides the whip you can use the "walls" of the arena, can't really explain in English (believe me I've been trying for a good 10 minutes), but if you make her lunge near the wall she will most likely associate it with the normal work she does whilst being ridden and thus will not fall in (I like th use the corner with especially bad horses in the beginning until they understand where I want them to be)
Do you lunge with a lunge whip? If she moves in towards you use your lung whip and point it at her shoulder, if she comes in closer tap her with it till she moves away. You can start by asking her to move away when the whip is pointed towards her shoulder on a small circle and then gradually increase the size. Also don’t move away instead be firmer with your whip if she doesn’t listen.