I didn’t expect to feel this way

I feel like I am failing my lo. She had her 1 month check up and her weight is not keeping up with the rest of her growth. She is just barely above her birth weight. The doctor wants me to supplement with formula. Of course I want to do what is best for her and logically I know this means I’m still breastfeeding and this doesn’t have to be long term. Emotionally though, I just feel like I can’t do the one thing only I can provide her and I am already falling short as a mom. My husband tries to understand, but I know it is also ridiculous sounding and he can’t completely. I’m just so sad.