losing my head in the air

Today I did an exercise of diversion which is an important part of the aviation navigation. It means that when the weather is getting bad and it seems to be impossible to complete the flight plan, we have to divert to another place to land. We have to do it just with a chart and pencil. We can't use a calculator or flight computer in this process. When I tried to do it, I needed to calculate like how many minutes required for flying 10 miles at 100 knots, and I couldn't do this easy calculation in the airplane for a while. I have once experienced the same situation in the deep water when I was taking a scuba diving course. I was taught that it was because of nitrogen in a high water pressure, but this time the air pressure was quite normal, and I think the reason of my failure was the mental pressure in the cockpit. I'm extremely getting nervous while flying and my ability is going down. It is my weak point.