Low breastmilk supply

Ladies, I need your help!! I feel like I’m about to have a break down soon. I went back to work when my daughter turned 6 weeks. I was able to pump 2x a day at work and have more than enough supply for her. However, now my daughter is 14 weeks and she’s been eating 3oz or more per feeding. She’s only been given breastmilk or breastfed so when we try to give her formula she just refuses to eat. I really don’t know how or what I can do to produce more to what she needs. I planned on breast feeding for a least a year so I’m desperate at this point.