Oh yes, I hear you! I've gotten spoiled with my shrimpy quarter horses. I kind of like being able to see over their backs. I about had a heart attach when I started taking dressage lessons and she threw me up on a nearly 18 hand thoroughbred! I'm terribly afraid of heights and that was a little too much for me
But it actually wasn't so bad. I got more comfortable with him as the lesson went on. The only thing I couldn't get over was his canter. It was HUGE! I felt like I was being run away with even though he was just going normal speed. I never got a chance to work on though. She leased him out to a girl at another barn, and I went back to riding her slightly smaller saddlebred. Who I think is closer to 16 hands.
Just take things slow and give yourself time to get used to the height. 16 hands isn't a huge leap up from 15.2. It's really only a couple more inches. Good luck!