Comment below with thoughts please!

Our daughter has lost weight and her pediatrician said that long nursing sessions don't benefit baby because, at a certain point, she loses more calories than she is taking in. She has reflux and the slow feedings make her feel better and not throw up as much, so she has self regulated to slow feedings... But I feel like my supply is lower because of it. The doctor suggesting adding formula to pumped breastmilk to make sure she is getting enough calories... But I can't get enough pumped to feed her. Any thoughts?