Help! Advice!

My little one is almost 2 weeks old. She was a a little early, born at 37 and 4. Nursing has been a struggle and we’ve had to supplement. We started supplementing with EBM and formula because she struggled waking up, lost weight, and was jaundice. She’s getting better at nursing, but I’m my supply isn’t great. She will nurse 15 each side, but then she’s still frantic. By the time I finish giving her a bottle the feedings are lasting an hour. I was told in the hospital to not let her eat longer than 40 minutes, I don’t want to cut her nursing attempts short but she is frantic and needs a bottle after too. Any tips or tricks? If she’s doing okay is it fine to let her eat longer?