Did you keep breastfeeding after going back to work?

I just returned to work and had been planning on continuing to pump and bf until at least 6 months. She is 12 weeks today and bf is going decently well. I work 12 hours for 3 days, then have 4 days off. I'm not pumping as much as she eats when I'm gone, by about 5 or 6 ounces. It's also hard for me to pump as much as I want to, I aim for 5 times a day (every 2 hours) but I usually am only able to do 4. I try to go for 5 minutes after my milk stops to increase supply, but it doesn't seem to do anything. Overall pumping at work is a hassle and I don't enjoy it. I feel selfish for wanting to switch to formula, but it's having a bad effect on my work status having to take so many extra breaks. I don't know what I should do, I feel too guilty to stop, but am having trouble continuing. Anyone else in the same boat?