Help.. left breast milk supply is depleting

I'm currently 4 weeks pp, and 1 week into breastfeeding my left nipple was cracked and whenever I breastfeed my daughter it would bleed. However, I got a manual pump and visited the lactation consultant and things went well afterwards.

She's almost a month now and I know that the supply will keep on increasing, but I started to notice that my left breast doesn't produce that much milk! Every time I would feed her from my left breast, she's not satisfied and full until I feed her from my right breast.

I'm currently pumping and in 20 minutes my left breast produced 25 ml, as opposed to my right breast producing 25 ml in 10 minutes.

I'm planning to exclusively breast feed until 12-18 months, and I would like to know how you were able to keep up your milk supply. I don't have a schedule I just pump whenever I'm free, and I'm a stay at home mom.