The vice is called "balking" and is well known. It means the horse is barn sour - doesn't want to leave its home surroundings. It simply doesn't want to go riding with you. Trouble is you are going to have to find out just why.
It is going to be a severe test of your horsemanship. Getting angry won't help. Being firm and positive might. But be careful about hurting the horse - it will remember. Be very careful with those spurs - they are not for punishment
First thing to check out is that the saddle fits, and the bit does not give pain either because it is too harsh or your hands are too heavy or rough.
Is there any justifiable reason why the horse feels that it is entitled to stop - ie some pain somewhere? If you isolate everything, saddle, mouth, teeth, feet, muscles then it is being cussed. Then you have to out fox it. That takes time and experience.
You have a trainer - it is just the thing to talk about with the trainer. If
the trainer is any good, then you are going to find out.
You say nothing about the history of the animal - you need to find out as much as you can about its previous history. You need to know what it had for tea this time last year and everything that has happened to it leading up to yesterday.