I haven't read all of the responses, and I am not a doctor or lactation consultant, but I do have 2 young kids and a baby. If I were you I would try pumping once in the afternoon just to see how much milk you are getting. If it seems like a good amount even though you don't feel as 'full of milk' then keep BFing her as much as she wants, and maybe add a little formula to her cereal to add extra calories.
If you pump and it seems like you aren't producing enough milk, then I would BF her, then offer her a few ounces of formula shortly after that to complete the feeding.
As far as yourself, try to make sure you are getting good nutrition, if you have a hard time making 'meals' then try to grab something like a piece of toast with peanut butter, a piece of fruit, a handful of nuts, here and there. Also keep taking your prenatals or another multi vitamin.
I've also heard that a beer a day (micro brews are better), or sprinkling brewers yeast on your food, can help with milk production.
Whatever you end up doing try not to beat yourself up over it. I always think of feeding my baby breastmilk like eating organic food. You know it is better, but even if it is half of what you feed her then that is half better than not doing it. And if you have to switch to formula 100%, you still got in 8 good months and she will be fine on formula too.