I don't have any concrete answers. Every child and situation is different and especially stressful this early in the child's life. My older daughter, now 4 1/2, would latch on and immediately fall asleep - from the time she was born until she was over three weeks old. She continued to lose weight all that time, and did not get back up to birthweight until 5 weeks old.
I remember how scary it was and how afraid I was. I had a home nurse that came to us twice a week to weigh her and to help me with different techniques (such as a special nipple to put over yours that makes sucking a bit easier) or a small "straw" thingy that I put pumped breastmilk in and then slipped into her mouth while nursing her (to get extra calories in her tummy...)
My milk supply did drop and I never could get it back. My nurse found herbs that helped most people get more milk, but it my case it didn't help. You could try them as well (Fenugreek and Brewer's Yeast). I had to supplement with formula, and it made me very sad as I had wanted to breastfeed exclusively. However, I came to realize that although breastfeeding is the best option, the most important thing is to get her fed and growing. I continued to breastfeed as much as I could.
She always was slow to gain weight.. At one point she was in the 95%tile for height, but only the 5%tile for weight. However, she is healthy and fine now and actually starting to gain more weight than she should.
I guess the only advice to give you is
1) don't be too hard on yourself. You obviously love your little girl and are taking the best care of her.
2) trust your instincts more than the doctors. If you don't feel she is getting what she needs, keep pushing until you are satisfied
3) know that even if she continues to have problems - there are further medical assistances that can be provided and eventually you will get her to grow and thrive.
I wish you peace. God Bless