Well, I kind of look at knitting as ambidextrous. Sure, you can hold the yarn in either your left hand (continental knitting), or your right (english knitting), but the basic motion of the needles remains the same, to some extent. People knit in all kinds of ways, really.
I'm left handed too, but never had any trouble picking up knitting. Crochet is a different matter altogether. I hold the crochet hook in my left hand, which totally messes me up for things like sweaters. At least I think it does. I just can't wrap my head around that one. With crochet, I know all the basic stitches, and can write my own simple patterns, but have difficulty with "reading" more advanced patterns written by other people. Go figure.
I think you'll probably run into trouble in doing more advanced knitting, things like lace and such.
It really would be worth it to have lessons by a very patient person (someone left handed maybe?).
In any case, it was rude of that lady at the yarn store to blow you off, and I'm sorry that happened.
I guess I'm not really any help to you, because I knit the regular way. Maybe someone else will chime in.