By posting my essay up on this forum, if turnitin.com finds it, will my essay be counted as "plagiarized"? Is there any way for me to prove that the essay is, in fact, my own?
Turnitin won't find out because their "bots" have been excluded from crawling our website. For more information you may check: turnitin.com/robot/crawlerinfo.html#excluding.
Is there any way for me to prove that the essay is, in fact, my own?
Yes, by posting your REAL name in your profile. That way the posted essay is associated with your name, which - in most cases - should be enough proof that you were the author of the essay.