L.Z.
Chances are, you are going to feel guilty no matter what anyone tells you - it's a motherhood thing. We feel guilty for what we aren't doing, what we are doing, everything! You are trying to make it through all the messages society gives you and your own inner feelings. I totally understand! I weaned my now-six month old when she was three months from breastfeeding because I returned to work, and I will always feel a little guilty about that. BUT, I am a teacher, and it is virtually impossible to find a free bathroom, have more than 5 minutes, and get all other work done throughout the day in order to pump, so I made the decision I felt, and still feel, was best for all.
Regardless, keep in mind that people who tell you about the health benefits of breast milk are correct, but may not work full-time or even part-time, making it easier for them to say that to you. It is not anyone's place to judge your decision. I have an issue with people who do that and don't know what your life is like day to day. As far as the colds go, your baby has been introduced to day care and would most likely have colds even if you were still nursing or pumping. My girls' doctor said they either field the germs as infants in day care or when they get to preschool and kindergarten; either way, they will get germs!
Finally, give yourself major credit for nursing for six months!! If I hadn't gone back to work, I would have done it for that long with both my girls. That is really good! Your baby will reap the benefits of that forever. Think of the quality time you will have with your baby NOT worrying about pumping, her latching on, or those issues. For working moms, that time is priceless and at a premium.
I hope this helps. Again, I understand the guilt - it is NOT fun, and I wish I had the answer to that... I doubt any of us do! You are doing great so be sure to give yourself a pat on the back once in a while!