My sympathy. Dyslexic DD sweated blood on hers, not helped by the fact she had an early deadline and was recovering from a major illness. I think disquisitiones is right though that for many courses people will only pay attention to PS's where the candidate is marginal. That said if you miss a grade or two in the summer you will want the University to be able to reread a strong PS.
As a non academic, inexperienced, mum what helped us was to think of it being scored. A bit like a mark scheme. Search University websites for PS guidance for your subject. Someone will have something clear (I remember Warwick's guidance being good for DS' subject.) Then follow it. If it says three quarters to be devoted to the subject and a quarter to extra curricular then do this. One para on why you are interested in the subject, one or two with evidence that you have taken this interest further. (Reading, lectures, essay competitions, relevent work experience etc.) Then one about non academic stuff. Sport, music, volunteering or other interests, ideally giving evidence of other skills, resiliance, leadership and so on, that will make you a good student, coping with the course and contributing to University life. Recheck that you have covered everything that a University says they have been looking for, so you score as many points as possible. I am hoping that as long as the grammar and spelling is correct and proper paragraphs are used, Universities expect a PS to look like they have been written by 17 year olds and will not be that polished.
One thing I did when DD struggled to get beyond the first sentence was to do a check list of things she might include, based on my knowledge of her interest and experience. Writing something very personal and very important was a huge challenge so a small amount of initial help with structure and content went a long way, as did using guidance from Universities as a template. In the end what she produced after about 17 drafts is far more individual and more interesting than my first check list, but she struggled to get started. The other area of difficulty was reducing it to the right number of characters. Again editing out duplications etc is easier done by someone further removed, though again I was consious of the need to retain her content and voice, whilst cutting down on characters. She also showed versions to others including her teachers and student brother, who were able to give feedback on how it looked to an outsider. What was frustrating for her was that she could see the problems but was wildly hoping someone would tell her what to write instead, which they did not.
There was a huge relief when she finally pressed the send button. Good luck.