Purchasing two pairs of shoes

I've purchased two pairs of shoes from Nine West, I normally take a size 4. With the first pair the lady that sold the shoes to me brought me a size 4 and the shoe felt good on my feet when I fitted them the following day when I wore them to work I felt it pinching my toes and I thought they might stretch. So I went back and bought another pair of shoes but this time I took a 4 and a half, only to discover these shoes are hurting me so badly I can barely walk. When I asked them at the store if I could bring them back they said no because I've already worn the shoes. To me this is unacceptable because Nine West isn't R200 or R250 their prices are above this. I need to know what they going to do with both these pairs of shoes that is killing my feet. I spoke to their head office who explained to me that they cant change it because it was worn already and that I have to aske Nine West to send it to a place where they will stretch the shoes. I don't want to hear this because I spent money at their shop and they are not prepared to assist me.