I have a nearly 3 month old daughter. I'm breastfeeding about 75% of the time and mixing in 1-2 bottles per day of either expressed milk or formula. I wanted to do formula every now and then to supplement vitamin D, and to make sure she would accept it - in case of a shortage. My baby is a little finicky with the bottle, but nearly always accepts the breast.
A close friend had her baby 2 months before mine, and started with bottles kind of early. Her baby basically refused the breast after a few weeks. Within 2 months, she wasn't able to stimulate enough production with pumping alone and is now (at 5 months) pumping once per day and doing the rest with formula.
My best advice to you is to do what works best for you and the baby. She will grow and thrive with your love & care - whether it's breast milk or formula.