You should definitely do what you need to do, I would also like to suggest (hope I'm not repeating anyone's advice), but I would like to suggest that you pump your milk before trying the formula, because some babies have problems digesting formula and your baby may not take to it and at least you will still have your breast milk back up for a while, even if its just pumped out and kept frozen/refrigerated.
Also one other possibility is that it isn't that the baby isn't eating enough so much as the baby may be comfort nursing. Its a very relaxing thing for them, and my son used to just wanna sit there and nurse and be close to me, even if he wasn't eating. See if the baby is really trying to chug the milk down or does the baby slow down and just sort of look around and maybe even doze off while staying at your breast. I hate pacifiers, but with my son, I gave him one when he stopped really eating and he would suck on that for a bit and be content.
Hope this helps, anyway you should never ever feel guilty!! You're out here trying to find out info and that shows you care a lot about doing things the best you can.