I'm in the same situation. I've read all these things on how NOT to write a PS or a Statement of Purpose...but they are all addressed to PhD applicants..I'm going to apply for an MA in Linguistics and I am writing both essays (PS and SOP). The first tells how I kind of rejected the discipline of linguistics...being a language teacher who writes, I just thought that linguists' deal was with spoiling the "mystery of language" lol...Then I developed on how that prejudice changed as I carried out research as part of some courses and was introduced to different views of language...and how I ultimately became more involved in the field of linguistics with some mention of activities outside my academic education which had an impact on my future interests...But overall, it's still a very personal text, and I use it as a way to put through my ideas and views on language, and I finally mention what I would like to study, but I don't dwell that much on that since I have my SOP to write..
On a related note..is it better to state two/three interests in an SOP or just one and develop it deeply? (As a prospective MA student I can't really say I have THE definite future project..)
I have been wirting both essays for like..2 weeks now, and I'm still not satisfied with what I've written x_x