Breastfeeding problem. What to do now.

I'm feeding my 16 days old daughter who was born preterm in 37th week. She latches very low due to which my nipples have become very sore and feeding is very painful for me. Secondly, she latches for comfort and doesn't sleep before 2 hours of continuous suckling. By the time she finishes, it's time for the next feed! I'm tired most of the day. She sleeps well at night, doesn't get up herself for 6 hours, so I wake her up and feed her after 4 hours maximum. Yet, once she's up, she doesn't sleep again easily. But I wake her up because I've been advised not to leave a gap of more than 3 hours between 2 feeds. I've checked using a hospital grade pump, my milk production is 1.5 Oz in half an hour. And so is her appetite per feed. When she was born, she was given formula feed for a 5 days, then I started feeding her. Now when I try to give top feed with the same, she refuses to take it. She throws it out of her mouth. Please suggest what to do now.