Mine's probably a doe. Before everybody freaks out and accuses me of copying, deer have always been my favorite animal from a very young age (causing much teasing from my friends) before I even read the books. I thought it would be a doe before I found out Harry's and was super excited when his was a stag. My happy thought would probably be a made-up memory. Right now the best thing I could think of would be getting my Hogwarts Acceptance Letter, which I suppose I would have already gotten (or a different school's since I live in America) if I could produce a Patronus, so the day I got my acceptance letter.
I think you should choose your animal! didn't a wise man once say "it is our choices that determine who we truly are" ;) but if you really want to determine your personality, just ask a friend what animal you remind them of, it'll probably be easier for them to tell you. My patronus is a Golden Hawk! My happy thought is Auburn winning the 2010 iron bowl :)))))