I feel like a failure

My four month old baby had formula for the first time today 😞 I’ve been loosing way to much weight I’m 5”8 and 115 I got told to

Supplement her during the day time to help with myself and I feel like such a POS mom for it she seemed to like the formula but my first one got breastfed for a year even though I got down to 110 with her but I never thought of giving her formula:/ I just don’t know what else to do people say oh eat more like I eat all the time even when my body doesn’t want to I eat healthy I eat junk food I drink water I drink soda! I’ve been to doctors and they just say it’s the way your body works! But no one understand everyone just says oh you look like like a boy again cause your so skinny! Like I’m sorry I don’t keep weight I’m not doing it on purpose I’m doing everything I can to keep my baby happy! 😒