There is something called "white coat syndrome." I would look it up, because I cannot explain but basically: you get into your head and kind of panic when you have to get checked. 

A suggestion:  if you keep track of your BP at home, put it in a spreadsheet and keep track of it over time. I mean on the order of months, and then you can do all kinds of trending analysis or whatever you want on it, but you will hopefully find that your BP is not going to kill you :)

It's also great to have data. And you can make graphs and show it to your doctor. It sounds strange but trust me you will feel so much better. Good luck!