In attempting to breastfeed all three of my children and only being successful for one out of three, I can say that I have encountered many problems and issues. Please don't take my failures as a lack of trying or lack of desire. In my attempts and with my success for my middle child (my first son), I've gone to a lot of La Leche League meetings and talked to my friends who were breastfeeding. We had very different issues.
One of your comments stuck with me as I read you message. You said that she tends to refuse once the milk lets down. A friend of mine had an issue very similar. Yours may not be the same but the solution is worth trying. She had a very strong letdown reflex. It made her daughter gag when she was eating because the letdown was so strong. She would pump (either with a machine or with her hand) just til her milk let down. Then she would breastfeed. Once the initial letdown was over her milk didn't flow quite as rapidly and she was able to feed her baby normally.
It could be that your daughter recently went through a growth spurt before this happened and was eating a little more (either in number of times or just a little more at each feeding). This could have caused your body to increase its milk production making the letdown a little stronger.
Or it could be something totally different. Your daughter is also entering the period of her life where she is really noticing the world around her. She may be distracted by the smallest of happenings and detaching herself from feeding so that she can explore her world and see what is going on. While some people recommend going to a dark quiet room, I recommend trying something else. While I don't know this will work for you, it is free and worth trying. Turn on the television or the radio/cd about 5 minutes before you are going to feed her and leave it on while you are feeding her. The music might work better. If it is the same or similar music each time, then she won't worry so much about what is going on around her. Also, she is less likely to be disturbed by any other sudden noises in the house. (My brother uses this for my nephew when he is sleeping. He is a light sleeper. He uses classical music.)
Here are a few things to remember... First, try to stay relaxed. They can sense when you are stressed and it makes the whole process harder (part of the reason I gave up trying to breastfeed my youngest). Second, if your daughter is growing and putting on weight, then she is getting enough to eat. Part of the issue may be that she is more efficient at removing the milk from your breast, too. If you are concerned about her weight gain, call your pediatrician's office and ask to stop by and have her weighed by the nurse.
Anyhow, I hope that this helps you out. Feel free to email me if you need to someone to listen to your woes or share your successes. I know it really helped to have someone to talk to with my breastfeeding issues. It made the difference between giving up with my first and making it through the difficulties with my second (I had milk supply issues with all three and an inverted nipple. My second child, a boy, was also a failure to thrive baby. Both the second and third were jaundiced and were supplemented to help work through it. My third child, second son, refused to breastfeed. He wouldn't even latch on after the first week. I kept trying for over 6 weeks.
I tell you all of this so that you know this... Relax. You are not anywhere near the worst case scenario of switching her to formula. But in looking at the worst case scenario, I want you to know that my daughter who was formula fed exclusively from 6 weeks on is now a beautiful 8 year old second grader who participates in the Gifted Program at school and has had a 4.0 in school for her entire academic career. So while you remember that breastfeeding is optimal and something you want to do. Also, remember don't feel guilty if you decide you need to switch her. She will do fine either way. (Please do not take this as an endorsement of giving up. I did my best for over 6 weeks with each of the two who didn't breastfeed to get them to breastfeed. I was also a bit depressed when it didn't work out.)
Good luck with the breastfeeding. I hope that my ramblings help you out a little bit. Also, I hope that you look for a La Leche League group near you. They were a wonderful source of support and knowledge for me.