The pain near your armpits is probably ducts that are either super full or the sensation of your milk letting down, the milk ejection reflex is really intense for some people. It is painful for me- like spasms of the boob.
If you feel any heat or hardness in that area it could be a clogged duct, which is common with a new nurser and oversupply. If I feel one I lay the baby on his back and nurse on my hands and knees over him. It sounds crazy but it will help totally clear all the ducts in that breast and prevent mastitis. Also massage the sore area in a hot shower or while pumping until you feel it soften and then nurse on that side as much as possible. I know you didn't say you have clogged ducts but it is good to jump on it if you start to feel that happening so it moves before becoming mastitis.
None of my kids open their mouths wide, petite nursers that they are, I hold their chin and bottom lip down to get them to open up bigger as they are latching on and it takes a couple of days for them to get used to that but it will work over time.
If your nipples still hurt while not nursing check for white in the cracks- it could be thrush ( weren't you on antibiotics for a little while?) My experience with thrush is shooting nipple pains when not nursing and putting them in the sun really helped me. A lot.
And I am drinking the fennel/chamomile tea. Lots and lots of the tea. Like a half gallon a day. It makes a huge difference for the little belly.
Cynthiamoon- I didn't let any of mine go over four hours without nursing, that was the recommendation of my midwife because of their blood sugar. After the first sleepy week it wasn't a problem.