I don't care how beautiful you are - or aren't - you are just going to suck in a department store dressing room. With the harsh, florescent, overhead lights shining on you in a closet sized space casting shadows over your body and the pasty neutral paint that blends into your skin tone - regardless of your skin color, there is no hope for a positive outcome.
Every blemish, every dimple, line, crease, bump or lump is magnified and negatively enhanced. I believe they use refurbished carnival fun house mirrors. And even if you do your hair really cute and put on perfect makeup - you are going to look like hell.
And because the experience is so awful, many of us wind up buying things that really don't look great on us. Because even a burlap sack is going to make us look better than we do standing in front of the mirror in just our underwear in a dressing room. Once we're home and try it on in front of our own reasonable mirrors, we can see that the item was "on sale" for a reason.
And you've heard a million times of the painful experience of trying on bathing suits. It's really bad - but not as bad as trying on bras.
And that's why we buy so many shoes.