Your nervousness is a mental issue and I’d guess that 99% of horse trainers are not equipped to help you overcome the mental issues causing your nerves and tension.
I agree that if you want to continue riding you will need to find a different trainer, likely with private lessons, who is well aware of your nerves and is willing to work with you. You need more direct attention than a group lesson program can handle right now, and more care with the horse. I do think many horses in lesson programs may be just fine for more riders but then act up with an extremely tense rider on board. You need a trainer willing to work with you.
But in addition you should also look into behavioral therapy that will help you develop skills to address your nerves alongside your lessons. This is not talk therapy! You need to research specific types of therapy around sports psychology, cognitive behavioral therapy, and phobia therapy. Make sure you talk to the therapist in advance about your issues and hopefully between working through the mental part with a therapist you can plan for skills to apply when you are riding with a trainer in a more supportive environment and then make some progress.
Good luck!