Hang in there. It sounds like things are starting to improve. For what it's worth, my son was actually 2 weeks late, and had many of the same problems. It was a struggle to get him to nurse for 10 minutes at a time. He lost almost a pound by day 5 (he started at 9lbs 11 oz, so this isn't as bad as it seems). He didn't get back up to his birth weight until he was two weeks old. He was almost 2 months old before we really got the hang of painfree nursing. Some of it was me learning how he was supposed to latch on, but a lot of it was just waiting until his mouth got big enough for my nipple. One thing I would do to help the pain was put breastmilk on my nipples, and then run around without a top on for a little bit.
Keep us posted.