Frustrated!

My LO is 11 days old and I've breastfed her the whole time. I've noticed lately her not getting full at her feedings and I hate that she's hungry! Last night I was up with her every 45 mins nursing and it was hurting so bad I couldn't take it anymore so i grabbed an enfimil sample bottle and she ate the whole 2 ounces in no time. I never really wanted to formula feed but I hate how she's not getting full and I'm starting to lose my mind with being up 24/7. Does anyone pump then feed with a bottle and supplement formula if they're still hungry? Needing some input and what kind of formula do you use?