Help: A Level issues writing essays

hi,
i just started my a levels in september and am really enjoying my subjects so far and learning about them. my only problem is that when it comes to writing practice essays, i do so badly despite good knowledge of the subject. recently, i had to write an essay in politics in timed conditions but when it came to writing it, my mind was moving too fast and too slow at the same time. i immediately knew what to write but when i literally put pen to paper, the words came so slowly. furthermore, i re-read what i wrote and it really wasn't my best work despite knowing what i wanted to say. this experience happens a lot and when i have to write an essay, i feel too stressed or overwhelmed to write and this manifests in (if i have to write it at home) procrastination.
if you have any tips please help and thanks for reading xx