In the beginning couple months of breastfeeding, I really considered quitting and switching to formula since we were having a lot of problems and it was really tough on me emotionally getting used to everything with the constant feeding, colic baby, etc. etc. I felt soo guilty for considering it, and even worse- I was too ashamed to admit to anyone but my husband that I was even thinking about it! I was really determined to breastfeed before having him, but after all the adjustments and the challenges, it would have been sooooo easy to just switch to formula. To be honest, one of the driving factors kept me from quitting then was just the fear of feeling like I had failed. In our situation, we kept with BF and now I love it and don't want to stop anytime soon. But I completely know that guilty feeling you're talking about!
My suggestion would be to set a small goal for yourself based on how things have been going- my goal was just to hopefully make it to 6 months with my first and then see how it went from there. That way it was a smaller, more attainable goal. When I reached 6 months, I said great! And I wanted to keep going, so I set a new goal for 8 months- now we've achieved that one, too! Nice, 2 goals down! My next goal is just to wait until his teeth come in and see how it goes...that way if it happens before 1 year and I decide to stop, I don't feel like I've "failed" that I didn't make one year...I've actually met my first goal and then even more. I would say set your goal for 6 months and then see how you feel- if that sounds too long, then set it at a week or two. When you make it, you will know that you haven't failed your expectations, you met them.
As my lactation consultant put it- the first rule of breastfeeding is to feed your child- whether by breastmilk or formula, bottle or breast! I feel it's been worth sticking with breastfeeding; however, I have friends who've chosen to use formula for different reasons, so I can definitely see BF doesn't work well for everybody, whether emotionally or logistically.
I think this is similar to so many different aspects of parenting these days- I feel like I am constantly having my parenting style critiqued by outsiders and feel guilty for things we choose to do or not do. If you go all organic- some approve whole-heartedly and some may think you're being snobbish paying too much. If you cloth diaper, some think you're crazy and some may tell you you're saving the world. If you let your child watch TV, some think it's acceptable and some may think you're killing their brain. And with BF on demand, I recently had someone tell me I feed him WAY too much and I'm making him form bad eating habits that will last into adulthood. What I've come to believe is that you have to make your own decisions for what you think is best all around and then stick by them knowing you are doing everything you can to raise your baby the best way for your family. I have my own reasons to breastfeed- but it doesn't mean they have to be your reasons, too.
Good luck and hugs!