Breastfeeding frustration

I’m having a really hard time with breastfeeding. I really want to exclusively nurse and then transition to pumping closer to my return to work, but my little one is struggling. She was born a little early at 37 and 4 and we had to start supplementing in the hospital. She’s now 11 days old and still having to supplement. She’ll nurse okay, but I don’t think she’s getting much from me and when I pump I’m only getting about half an ounce. I’ve seen a lactation consultant, who said she was just acting like a preemie, but I’m so discouraged. Anyone else have experience and their baby got there eventually??