You will NOT be a bad mom if you choose to quit breastfeeding or go back to work or both. Many people will tell you that you are, but that's more a reflection of their own insecurities, rather than what's right. I've been a high school teacher for 17 years (starting my 18th year in 2 weeks) and I've seen horrible, rotten kids come from both SAHM and working moms, just as I've seen great kids come from both environments. You need to do what feels right for you and your family.
For me, I never aspired to be a SAHM. I took extended maternity leaves (6 and 9 months respectively) and I was thrilled to go back to work each time. I found a wonderful in-home day care environment, three blocks from my school, and I left both my sons there without a backwards glance. And when I did miss them, I ran up the street during my prep for a quick hug, smooch and cuddle. It was great. I never felt like I was missing out because the first time you see a milestone, it's just as exciting as if it were the actual first time. (Did that make sense? LOL)
As for breastfeeding, I had every intention of nursing until my sons were six months old, but biology conspired against me. I have flat, inverted nipples and it was sooooooooo hard to get my sons to latch on. With both, I worked with lactation consultants, me pediatrician, I even called La Leche League for guidance. Ultimately, I followed my pedi's advice: feeding time should be a bonding experience, not one filled with frustration, anguish and pain. I switched to bottles, pumped, and by 3 months, my sons were on formula. At 12 and 16, I can tell you they were not negatively affected by this choice (or that of being a working mom).
Bottom line, happy moms make happy babies and families. Follow your heart and you cannot go wrong. Many people, on this site and other places, will try to change your opinion or make you feel bad or, in some cases, support you. I look at it this way (as the history teacher I am): my Grandma, a Rosie the Riveter during WWII, did NOT have the choice about whether or not to be a SAHM or a working mom. The society she lived in dictated that to her. But, she worked hard so my mom and her granddaughters could have that choice. My mom was a working mom out of necessity. I was a working mom out of choice. My sons' wives may choose to be SAHMs. Whatever any woman chooses is HER choice; NOT something open for debate or ridicule. If we truly believe that women are free to make their own choices, then this must be one of them. No one philosophy has a corner on the "best child rearing" market.
Good luck.