Hi N.,
Sorry to hear you and your little one are experiencing frustrations with feeding....however, please try to be patient with her (I'm sure you are!) AND with yourself, because it's such a basic need and such a time of nurturing and bonding, if that makes sense - it's not meant to be something to just rush through and get over with, any more than our meals are supposed to be for us to be healthy. Make sense? Also, babies change as they grow, so some of the variation may be normal.
I'm sorry also that you were not able to continue nursing - you didn't say why, but I wish it had worked out differently for you and your baby.
You might want to consider that she could be experiencing infant reflux (and yes, it can be "silent reflux" which means they have discomfort, pain and burning without actually throwing up most of the time). The sitting bolt upright but still being hungry makes me think of this. My daughter had reflux in her infancy, and hers was mostly the "silent reflux" (doesn't mean SHE was silent, there was often a lot of crying and fussing!). Until we diagnosed it and then found a ped who actually understood and would help us treat it (she needed meds to control her discomfort as well as my being aware of positioning etc.), we had lots of feeding issues and stress. The on-again, off-again feeding your baby is doing seems very typical of reflux feeding to me - it would be worth checking it out and watching for any other symptoms, and talking with a knowledgeable pediatrician and/or lactation consultant about it. Not sure if you would be interested, but you could relactate and nurse your baby again and an LC could help you do that.
One thing we needed to do for our daughter was to feed her at an angle rather than held horizontally or "flat" (propping with a wedge pillow so her head was elevated helped a lot). Also we needed to hold her upright for about 20 minutes following a feeding to help prevent the refluxing from happening. I realize your daughter may not have this (and I hope she doesn't!!) but I'm just tossing out these ideas in case she might. There's lots of info on the Web if you search under infant reflux and here is one good mom-run webpage: www.pollywog.com. You could see if any other symptoms remind you of your baby - frequent hiccupping, for example.
One last thing re the bottles - when we had to supplement for a while when she was tiny, until the reflux issue got resolved so she nursed better, the bottles that worked the best for her above all others were the Dr Brown's bottles (the wide necked ones). They really do work to reduce any chance of intestinal discomfort and gas, and were better even than the Avent bottles (which are what we used prior to that). For me, I REJOICED when the day came that I could put all that bottle paraphernalia away for good and not have to fool with it anymore (she was a champion nurser by about 2.5 or 3 months of age, yay!) but I know that it's not the same for everyone. :)
Just take or toss - hope something helped here!
Enjoy your sweet little ones (and not-so-little one!),
God bless,
S.