Back in year 10 and 11 I also felt really anxious about taking my GCSEs, especially when I thought about the exams or how I would feel right before an exam. My year 10 experience was terrible, and I was constantly doubting my own abilities. I think I got better at handling pressure as time went on and slowly got into my own rhythm of things when I got into year 11. It's okay to feel worried, but don't let it consume you completely, since you still have so much time. I've also adopted a much healthier mindset on myself, where I prioritise improvement over hard grades. There are many things that time and experience will teach you that I can't explain through a small block of text.
A lot of this is my own opinion or approach but, I basically don't think about my exams at all (even though I'm a year 12 studying for A-levels); maybe I don't think about exams enough, but I still find that my mind was a lot more at ease in year 11 compared to year 10, even though year 11 was closer to doing my GCSEs. I just focused on my own mistakes and faults, and revised like crazy.
I hope you start feeling better soon!