This is quite well written, though I am not clear about who is the intended recipient! It is obviously not a UCAS PS, since there is a reference to KS3. I therefore assume it is designed to be a statement of your achievements, qualities and aspirations as you currently see them.
To this end I have made a large number of changes, not only in the wording, but also in the order, so that it all reads more logically, with common themes now grouped together. I have added a critical final sentence too, as well as removing all the more negative observations, and one or two rather trivial ones. It is overall a little shorter after all this.
I hope you like the amended version.