Annie Ferguson Muscato wrote openly about the "pain and tears" she has gone through trying to breastfeed her daughter.
In an open letter to the stranger on Saturday 16 April, Muscato explained why she now believes "fed is best".
"You didn't need to tell me 'breast is best' as I was buying a can of baby formula, because I already know," the mother began the post.
"I know that my husband and I excitedly took the four-hour breastfeeding class when I was pregnant," Muscato wrote.
"I know that we saw a lactation consultant before we took her home, and again a few weeks later.
"I know that we struggled at first. That some nights we both cried together. That my dear friends swore it would get better.
"I know they were right, and it did."
Muscato then explained that every time she tried to breastfeed her daughter, she would "scream writhing in pain".
"I have held my child, my baby, while she screamed for hours - one day for eight hours straight," she continued.
"We have been to see the paediatrician at least twice a week since she has been born.
"I tried cutting soy, dairy and leafy greens from my diet to make my milk more digestible for her.
"I have pumped enough to have hundreds of ounces of breast milk in my freezer even though she will likely never be able to eat it."
Muscato said she then tried her daughter on "hypoallergenic dairy protein free formula" and she stopped crying, began sleeping and even smiled.
"I cried because I thought breast was best," Muscato continued. "I thought my body failed her. I thought she wouldn't be as healthy on formula.
"I know you think I must not care or I'm lazy but you are wrong.
"What I know that you don't is that breast isn't always best. I know happy, healthy baby is best. I know fed is best.
"So, dear stranger, remind yourself that fed is best and smile because it means someone loves their baby enough to do what's best for them."
Annie Ferguson Muscato and her husband
Muscato asked people to share her post in the hope that it may reach mums who "need to see it".
It was shared nearly 45,000 times in six days, encouraging other mums who have struggled with breastfeeding to speak up.
"When I had my first son I was devastated that my milk was not coming in," one mother wrote.
"In my baby support group, another mother told me that I 'didn't try hard enough to do what was best for my baby' and I was 'selfish'.
"Those words stung for years. Many years later I know the problem was her and not me."
Another person wrote: "As a failed twice at breastfeeding mum, I can totally relate.
"My milk never came in for my first, not for lack of trying, and I felt defeated when I had to buy bottles.
"Well done for proving 'fed is best' sometimes."