Frustrations

We've been going into the doctor's every 2 weeks since she was born just because of her slow weight gain, and this last time (Wednesday) she only gained 4 oz in 2 weeks. So, the doctor has me pumping extra and offering her that after nursing, which even after a 40-minute nursing session, she'll gulp 3.5 oz like an African refugee baby! Of course, he initially wanted me to do formula, but I told him I'd rather pump, which he said was actually preferable to him too (so why didn't he recommend it in the first place?!?). He thinks that's why she's crying, because she's still hungry. I don't get it though, because she nurses all the time!!!!! Seriously, some days she nurses for 30 minutes, sleeps for 30 minutes (as I hold her), nurses 30, sleeps 30 -- all day long! I can't understand why she's not getting enough. The doctor said maybe it's that she is just a really inefficient nurser and doesn't get that hindmilk after the initial let down. I don't know what it is, but it's stressing me out big time! Zack stayed home from work Friday so that I could pump after each of her feedings, and he could help take care of her. He's so great -- I can't imagine doing this without him. Single girls really make it hard on themselves! There's a reason God says marriage first. Anyway, even after her feedings I can still get an ounce and half or so, which makes me wonder why she isn't emptying that out... AHHHHH!!! I'm so confused. I had NO IDEA breastfeeding could be so horribly hard. I really want it to work out, and for us to get it figured out so that I don't have to pump all the time, but can just nurse, but I'll do whatever I have to.