breastfeeding/pumping struggle

I work full time so the majority of my days are spent at the pump instead of breastfeeding. I am thoroughly exhausted. Sometimes I have to pump before work, 3-4x during work, and once before bed just to get the 12oz she needs for daycare the next day. It makes me so miserable and crabby. I'm considering formula supplementing but I feel so guilty about it. Why?!?! Do any of you do a combination of expressed breast milk and formula? Have you noticed any changes in your baby when you introduced formula?