Returning to work and breastfeeding

I have to go back to work (5 days a week) in May when my daughter is 4 months old. She is exclusively breastfed, I make enough milk to feed her and her weight gain is great but I find it difficult to pump enough for a feed.

She has 80-100mls per bottle but between feeds I can pump maybe 60mls max, I have breastmilk collection shells that collect around 100mls throughout the day.

Will I have to end up giving her formula? I really wanted to exclusively breastfeed but at this stage I really don't think it'll happen 😭