My 6 wk old daughter is very constipated. I'm feeding her Enfamil Gentlease. Is there something better?

I have tried Karo Syrup, massage, exercise...you name it...nothing has worked. I breastfed until my supply tanked, so now I am looking for the best formula possible. I want something as close to breast milk as possible and don't want to lose any minerals or vitamins (iron etc.) If I don't have to. Help!