felt like a total failure as a mother
I had the exact same problem. My first son was born 3 weeks early at only 5 1/2 pounds. He nursed as much and as hard as he could but was losing weight because he wasn't getting anything.
The doctors threatened to hospitalize him if he dropped below 5 pounds. When he reached about 5 lb 2 oz we tried formula just to keep him out of the hospital.
He gulped it down and started gaining weight right away, so I knew that was the solution for him. I too cried and felt like a miserable failure as a mother, because really, who can't breastfeed?
All the lactation information I could find just said to keep trying and that anyone could do it, so I felt like a total failure as a mother. Also, my breasts are large - I wear a G cup bra - and I thought surely they should be able to feed several babies, while my sister who has small breasts was able to feed her baby with no problem, but it was not to be.
My second son was full term and larger, so I determined to try harder this time, thinking maybe I just gave up too soon the first time.
We virtually stayed in the recliner with him nursing till he fell asleep in exhaustion and then waking up hungry again a few minutes later.
Again I finally gave up and went to formula. Both are now healthy young men and I know I made the right choice, as hard and crushing as it was for me. My mother told me once that one of her sisters, my aunt, had the same problem, so apparently it is not as rare as I had thought. It's just never something the doctors or anyone else ever tells you could happen.