Vistaril is an allergy med, but it is also used for anxiety (and does not have the addiction/dependency issues associated with benzos) and even for nausea. It's one of those meds that has a variety of uses, even though it is primarily classified as an antihistamine.

My son took vistaril for anxiety and it worked well. He didn't find it caused him any side effects. I've been given vistaril for nausea and it knocked me out for hours.

But you said that antihistamines trigger depression for you, so putting you on an antihistamine for anxiety seems like a decision that wasn't thought through. You might talk to your doctor about this.