Breastfeeding troubles

I am 5 days postpartum, I delivered two weeks early. I chose to exclusively breastfeed from the beginning, however, my milk supply is very low. It doesn’t seem to satisfy his needs. His weight dropped more at three days old and his jaundice went up. Because of this I had to start supplementing formula. I still breastfeed before supplementing formula and self express aside from that. But I am finding it hard to not feel discouraged with breastfeeding and like I am not good enough as a mother because I can’t exclusively breastfeed. Was hoping for some words of advice or encouragement.