My horse goes wild on the lunge, how do I fix it?

Hello! It sounds as though the two of you have not yet found the communication for going forward and slowing down or halting. Start by being consistent with your aids, so she stays attentive to you and so you apply the same aid and the same reinforcement every time. Begin in walk and stay there until it is genuinely solid. Be content with that. Then move up to trot. Once she will halt or slow down on your signal without any reinforcement, you can bring the canter back in, just a stride or two, then slow down, building up the number of strides over time.

Good luck!