This happened to me a lot when I was preparing for GCSE's. when we were set an essay, my mind would go blank or I would just hate anything I tried to write and I would just start crying. I worked out it was because I wanted to do well so if I thought my essay was going badly, or if I couldn't think of a perfect first sentence, I immediately panicked and wouldn't write anything because I felt like it would be the end of the world to hand in an essay which wasn't my best.
I don't know if you're going through anything similar to me but there are a few things which have really helped me get better at essays which you could try!
Firstly, It's really worth speaking to someone about what's going on. Your English teacher, form tutor, your parents, school councillor, friends, anyone who you trust. My English teacher was really helpful for me because she'd taught a lot of students with similar stress towards essays so knew the kind of things that would help. I know it might feel embarrassing but I promise this happens to a lot of people.
Secondly, writing out an absolutely bare bones structure for your paragraphs. This means that even if you do start stressing out, you have something to rely on, even if its really basic. It definitely won't lead to the best essay ever but it will be far better than not handing anything in! For me, this was using TEAL or PEEL structures to just write out 5 or 6 sentence paragraph plans.
For example:
It could be argued that _____ .
This is shown when _______ .
Here, the author uses _____ to emphasise _______.
Therefore, __________.
Overall ______ .
Filling in the boxes is a lot easier than staring at a blank page trying to formulate something new. I found this really helpful because it got me through panicky moments. Once I started writing, usually I felt a lot better and I could go back through and improve and add to the paragraphs. If you have an understanding English teacher, they might even let you bring a sticky note with something like this written out on into tests with you.
Finally, try and take the pressure off yourself a bit. I know this is easier said than done but its worth reminding yourself that you're only human and you will write some bad essays. That doesn't mean you're awful at essays or stupid, that just means you're human
I don't know if any of that will ring true with you but I hope you can find at least some of it helpful! Best of luck with your essays!