Don’t know what to do

My baby is almost 9 months and she’s obviously skinnier then most baby’s .. she’s formula fed (soy insomil similac ) and she eats a lot of purée and some solids but she won’t eat much and it’s becoming so obvious how skinny she is and it’s making me feel like a horrible mom. I try to offer her splice and she just won’t take them at all and I’m getting tired of being rejected time and time again. I don’t know what to do I can literally feel the depression from this coming on.