Trying to write a "personal statement" for a college entrance exam is difficult; and it does not get any easier as life goes on. Just look at the statements the tutors use on WyzAnt. They did not just sit down and write out a statement, willy nilly. Most of the statements are carefully crafted pieces of persuasive writing.
After all, all writing is persuasive.
The 'trick' is to think impersonally and to give yourself plenty of time for re-writing. Sorry, not much a 'trick' but then, good writing takes practice. And if you need it, a good tutor/editor.