I think the majority of guilt over the inability to bf comes from the reactions of other mothers.
When I gave up trying to bf I did so without so much as an ounce of guilt. My baby was hungry, he needed feeding, and I couldn't do it myself and wasn't going to beat myself up or starve him in the process. So I gave him formula and never looked back.
It wasn't until I joined mn that I came up against this concept that women felt guilt over not being able to bf, and the comments from smug, self-righteous women who constantly trotted out the line that not being able to, or in fact choosing not to bf was in some way not doing the best for your baby. Where I was told by a so-called (well respected on here) professional that I must have been mistaken over my claims that I did not produce milk as this is so rare as to be practically unheard of. Where i have seen threads from people expressing their sadness that they saw a mother in a supermarket either with a newborn and a bottle or buying a tin of formula. And the list goes on.
Frankly most of the guilt that non breastfeeding women feel has been inflicted on them by the reactions of some of those women who were fortunate enough to be able to breastfeed.
Obviously the majority of breastfeeding women are not like that, but there is a far too vocal minority that do the image of breastfeeding mothers far more harm than good.