Maintaining weight while breastfeeding

Ok so let me start out with I've struggled with weight gain my entire life, I've always had a BMI that was classified as underweight. I had a very healthy pregnancy, but by two weeks postpartum I was at an average healthy weight for my height. Ever since I've been loosing weight like crazy. I'm at an all time low, similar to my weight at 14 years old. I can't keep weight on for my life and I really want to continue nursing but it's taking everything out of my body. Recently I've been just barely producing enough to get bottles for daycare even with a midnight pump session. Any other mamas have advice for keeping the weight up even while breastfeeding?