trouble weaning from formula

Does anyone have some advice for weaning a baby off of formula?  My daughter is 13 months old and eats VERY LITTLE table food.  I actually had to put her on vitamins with iron because she was not getting enough nutrients. She was gaining weight, but her iron was low. I have tried to give a big variety of foods, but she is rarely interested.  I do not want to take her off of formula because if she needs vitamins WITH formula, I can't imagine what would happen if I stopped that.  She will only sit in her high chair for about 5-10 minutes and puts up a terrible fight if I try to get her to sit down. I have tried to let her feed herself,  I have tried to feed her while she sits on my lap, feed her while she walks around the house, etc. She will take a few bites and wants nothing to do with it. She even seems to be drinking less of her bottles.  My mother is telling me that I am creating a bad habit by letting her walk around and eat and not making her sit in her high chair, but she fights me something aweful if I try to keep her in.  She isn't eating meats or vegetables.  I can get some pizza, pasta (noodles from chicken noodle soup), bananas, meatballs, cantelope, gold fish, carrots, yogurt, oatmeal, ummm....that's about it and even that is only once in a while.  And that is usually only a few bites.  Should I be worried?  Not sure what else I can do.  Any thoughts why she would not be interested in eating?