Breastfeeding help..

Okay so my DD is almost 6 months old. I breastfed up until about 3 months ago. She wasn't really gaining weight and her dr told me she recommended that I switch to formula. I tried to breast feed & formula feed but was so discouraged, I gave up.. I really want to breast feed.. I know it's best for my DD and I've been upset about it ever since I stopped..