I have a vet-shy mare as well. The key is to stay calm around her. The vet must be confident and not let her sense that something is amiss. I must maintain the same attitude, very no-nonsense. Don't get angry or upset or scared, just remain calm and do what has to be done. Use common sense.
My dad's gelding is an absolute nutter around the vet. A few times ago, when she had to examine his head, he backed up into a corner and started to strike out and rear--completely unacceptable, but he truly was terrified, so you can't go beating a horse for that. A twitch used to work for him, which is I'm assuming what the vet used on your horse, and if you can get away with that, great! But it no longer works for this gelding.
Instead, we decided that I would pre-medicate him. I don't know if it's available in your country or not, but they now may a Dormosedan oral gel, which will sedate and calm your horse before the vet gets there. You give it just like dewormer. However, I am comfortable giving IM shots, so now I get the drug from the vet beforehand and give it to the gelding half an hour before the appointment. We've only done this once, but it worked like a charm. He's still not fun to give shots to, but he's far better behaved for me than the vet, and while he acted up he was by no means dangerous. Then, drugged, he stood calmly and quietly for his eye exam, with no fuss at all.