Go to your GP. You may be suffering from an anxiety disorder. There aren't any quick fixes or cures, but there are things that can help. You may find it useful to learn breathing techniques and meditation can be helpful for some people. The headspace app has some decent guided meditations.
Talk to someone at your school too. If you have a counsellor, they can help. If not, try to talk to the SENCO or your form tutor or head of year.
GCSEs are not the be all and end all. You can always resit them, and there are various vocational courses at FE colleges. You may find it reassuring to apply to one as a backup plan.
Long term, you may need help from CAMHS, counselling or psychotherapy might be useful, or CBT. You may well need to fight for this though. You can also have a look for support groups. I attend one on a long term basis, and I find it enormously useful as a management technique. Art groups can also help.
At one stage, I was as unwell as you, although for different reasons. I now have a successful career in mental health, I'm in a very happy relationship, I live in a tiny cottage that I love and life is good. You can get through this.
Feel free to message me if you want to talk.