Why feel guilty? Look, I know there's a non-stop mantra of "breast is best," and I suppose that if breastfeeding comes easily to you and your baby, then breast is best. But when you're having trouble, it's painful, baby isn't getting enough to eat, etc, then breast isn't best. I mean, seriously! The point is to feed your baby so she's full and can do all the growing that she needs to do.
Babies who are totally formula-fed are no worse off than those who are totally breast-fed. (Disclosure: both of my daughters were exclusively breastfed for the first 5-6 months, but it came easily to us, so we were lucky.) I read a study (peer-reviewed) recently where they actually found that the benefits of breastfeeding have been vastly over-stated. There's a small benefit in terms of short-term health (ear infections and the like), but long-term, there's no benefit one way or the other. I know the La Leche moms on here will go absolutely nuts reading that, and I'll admit I was quite shocked to read it as well, but you can't really argue with scientific fact.
Anyway, the point is, you're feeding your daughter breastmilk, so ultimately, she will receive whatever benefits that may give her (it makes no difference whatsoever if you feed her breastmilk from your breast or from a bottle). And if you switch to formula, she will be absolutely fine doing that as well. She may be fine going back to breastfeeding after having been bottle fed - we did both (I worked full-time). Please, stop feeling guilty about this. There is no reason to feel guilty. You are doing a great job.