Formula milk

Hi.. I only breastfeed my baby for 1 week after delivery.then I introduce to formula milk(similar total comfort) .. aftr 2 month I try changed to similar lactose free formula.. but my baby don't want to drink it at all... so I continue to similar total comfort.. so aftr 6 months I taught to changed coz she having constipation , so I try (firsolac...enfalac.. and similar step 1 ) all of it I give 2 days gap before change but unfortunately all of this dont suit her (having diarrhoea and vomiting vr frequently) then hmmm I changed it AGAIN to vr 1st milk (similac total comfort) .. So I wan solutions how to cure her constipation because I dont think so I can change milk powder for her.. PLEASE HELP ME