I tried to look this up in previous posts but no luck, so I apologize if this is repetitive.
Anyway, my daughter is a week old and - luckily - has made breastfeeding pretty easy from the get-go. However, the last two days she has been doing something different. Typically, I have to wake her to feed her. Very rarely does she wake to eat on her own. So, I try to wait to do it every 3 hours instead of 2. When I feed her, she will go for at most 5 min per side and then pull away or fall asleep. I assume she's full, so I'll put her down. Then about 10-15 minutes later she fusses and will eat for about 2 more minutes. She does this for a stretch of a couple hours, then she'll have a great round where she eats for about 20 minutes total and is out like a light. This is especially tough at night when I really want to feed her and go back to sleep but she makes it into an hour long session with only eating for, at most, 15 minutes of that time.
I know this is new to her, and me, but I was just hoping some veterans might be able to give an insight into what's going on or what might help get through this hump. 
Re: New to BF and frustrated
Ive only been feeding for 5 weeks now. At 1 week old, my LO was feeding every 1.5-2hrs for about 15-20 min total. I was told not to let them "snack" and do everything you can to keep them awake to continue feeding. My LC said to rub their feet, burp intermittingly and do a diaper change in between sides. My LO would always poop while eating so I quickly learned not to change his diaper before feeding him. Now at 5 weeks, he eats every 3 hrs and I do my best not to let him snack.
I'm of the mindset to watch the baby not the clock, too.
Were you medicated at all in birth or now? Medicated mommas can pass drowsiness onto baby, hence needing waking for feeds. If you weren't, consider NOT waking her and waiting for LO to tell you he's hungry. DD only cued me to feed her every 3-4 hrs, even as a newborn. I worried at first, but her poos were good and weight gain was too.
Keep on! It could all change in another day!