How can I stop exam stress and crying from taking over my life?

I'm currently taking my GCSEs at the moment, and am in year 11. I took 4 GCSEs last year and got 3 A*s (french maths and textiles and an A (music). There's now a huge amount of pressure to do well this year too. I sat exams for separate sciences and thought I had done so badly that I honestly didn't believe I got a C in biology or did well in physics either. I cried for nights on end because of this and when I found out I'd got 90% in biology, 94% in physics and 100% in chemistry I was so shocked. I still get so stressed out now, and haven't been able to stop crying about the fact that I got an A for my physics coursework and not an A*. What's worse is that I know if I don't get 8 A*s and 2 As in my GCSEs I won't be happy either. I feel like my mood and well being centres around how well I do in exams, and like if it's not an A* (in most cases) it's not good enough. I would love to be a doctor when I'm older but how can I stop being so obsessed about grades?