Re: Trouble gaining weight
She's within the normal 10% loss but she should be pretty close to regaining her birth weight if not already be there now. The problem with supplementing with formula is that every supplement given affects your supply because that's milk not being removed from your breast and it can start a vicious cycle of low supply. The better solution is to pump after every feeding and supplement her with your own milk. It's a pain in the butt but should only be temporary.
The better solution is to figure out what is causing your DD to not gain. Supplementing her only treats the symptom but not the cause.
What is your nursing like? Do you offer both breasts? How long? How often? Does she come off or do you remove her? Does she seem satisfied after nursing? Are you having any difficulties with latch or pain? Have you been supplementing her without pumping?
I understand how frustrating it is. I went through something similar.
E, wife to my rock, S 2009 Mama to DD 1/13