C.,
I completely understand the feelings you are going through. It is hard to give up nursing, especially when you feel like it is the best thing for your child. You really have to work through those feelings and make the choice that you feel is best for your child. I had a child who was not gaining weight with nursing and had to start supplementing as well. On the flip side, she still did not gain weight well with supplementing or with bottlefeeding alone, so there was more to it. She ended up having many health issues. I am not trying to say that your daughter does, but you do need to keep checking her weight gain even with the formula to make sure she is gaining with that.
I also would suggest contacting a lactation consultant if you want to keep nursing. They can help you with encouraging your daughter to nurse, and increasing your supply.
I also have a son who I chose to stop nursing at about 7 months because it was just too much of a struggle to get him to want to nurse and it would just make him upset. It turned out I was pregnant, and that may have been part of the issue. I also have a friend who always has to start supplementing at about 4 months because her milk supply drops. Sometimes you can do different things to encourage your milk supply and stick with it, but it does come down to how much effort and stress you want to put into it.
One thing I tried with my second daughter (though it is really hard to get the hang of and she was not a good enough nurser for it to work) is a nursing supplementer. It is a bottle that you put formula in and then run a tube down to your nipple and have them suck on that at the same time as nursing. They get the supplementation but are still nursing to get it. You can ask a lactation consultant about where to find one if you want to give it a try.
Anyway, I just wanted you to know you are not alone, there are many other women out there who have dealt with this issue.