First, let me just say kudos for pushing through all of the problems and still trying so hard to breastfeed.
I have heard of a way to supplement by using a tube that you put on your breast, so they still latch on and "nurse" but will be able to get formula or pumped milk through the tube. That way she would get more.
Also, are you having problems with the latch? If she isn't latching well, she wouldn't be able to nurse very efficiently, which would lead to a decrease in your supply.
My son had latch on problems at first due to a tongue-tie. It took him an hour to nurse and he had slow weight gain. After trying everything the doctors and lactation consultants could think of, we had his frenulum (under tongue) clipped. It wasn't bad at all, and he had an immediate improvement.
If you haven't talked to lactation consultants yet, you should try calling one, they are often better about figuring out breastfeeding problems than the pediatritian. Good luck and I really hope you can figure it out!