I'm also stressed by GCSEs, however while the work gets harder at A-Level, it will all be in subjects that I like and enjoy, so I'm fairly confident that while I'll be stressed, that I will be motivated to work and do well.
Some GCSEs I'm taking are so tedious and boring, or I am really bad at them(Spanish!), but I have to do the work anyway and hope for the best. The fact that some universities ask for a modern language makes it more stressful as well.
It's not even the exams that are stressing me out, its the bloody coursework :/