More breast milk

Now that baby is getting older she is obviously eating more. I can't produce enough milk! It's really starting to get to me. How do you get more? I have tried mothers milk tea. I'm pumping maybe 5 ounces a session which is what she eats in a sitting and is usually still hungry. At work I only get time to pump once a day, maybe twice if I'm lucky. So in a 9 hour day I may get 2 5oz bottles worth but she's eating way more than that. She's having formula now every day regularly. Which isn't a bad thing, but is it bad to mix formula and breast milk? I don't know why this is so hard?! I thought your body was supposed to know how much to feed? Help!