How to increase my breastmilk supply?

My baby was breastfed for the first 2 weeks of life, but I had a very difficult birth and had a hard time feeding her, so I bought a hospital grade Medela pump, and began to exclusively pump from there. My baby is now 4 months old and I'm starting to lose my milk supply! I used to pump a good 8 oz every 4 hours, now i'm pumping every single hour and a half for about 20 minutes (pumping both at a time) and when it comes time for her to eat, i'm always almost always half ounce short (she eats 4oz), or sometimes worse. I had to give her 2 oz of formula a few days ago because I simply was not making the milk. I was told by a nurse to try breastfeeding, so I have been trying for about a week, but my baby is just not having any of it. She cries every time. So I bought a nipple shield hoping it would be a bit more like the bottle, and she cries less with that, but still isn't interested. I've also been using bottles that are more naturally shaped in hopes that this would help her breastfeed again. I don't know what to do! I've been eating a lot and drinking SO much I feel like i'm going to explode. What can I do to get my milk supply back up? And why is it so low now? I know of a pill that has a possible side effect of increased milk supply, anyone had any luck with that? Anything herbal? Thank you!!!