Breastfeeding not going well

Brandy
Baby is 16 days old. I was breastfeeding only until we found out at our first Pediatrician Appt that she lost more then 10 percent weight. So I had to supplement with breast milk bottle feedings or formula if I was not making enough milk. Things have been much better since then but I need to get away from supplementing. 
Today I had an appt with the lactation consultants and things seemed to go well. I was hopeful. I got a different size nipple shield which seemed to help her latch and within thirty minutes she was able to feed and get 2 ounces. She was content. I was given some helpful tips and told to stop supplementing as she should be able to get enough milk from me. 
Since we have been home it's been terrible she feeds for over an hour and still doesn't seem satisfied. I am supposed to pump after each feeding but that's not happening because she barely rests or gives it time before she wants to eat again. 
I feel like I can't continue this. I feel so defeated.