Thanks so much for the replies!
When I was taking lessons (and would ride in bigger saddles) my instructors never had to tell me to correct my position (I think the most was just "keep your heels down")
And since I got Vega and then Montana and I would see pictures of myself, I just looked horrible.
When I got my saddle, I first sat in a 16" and then the 16.5" and the person said 16.5" was fine (but they also told Tom he'd fit into a 17" english saddle, yet when we went to a different place, they said he would be more comfortable in a 19" and that 17" would be WAY too small) So i'm convinced that that tack store didn't have a large selection and sold me the 16.5" because that was all they had.
I was fitted, just recently, for a 17" but I'm nervous that I will outgrow the 17" and will need a bigger saddle. Would it be OK to get a saddle that is half inch too big? Or should I just go with the 17"?