Try to gently hold her chin down as she comes on, with one of your fingers.
Model a wide open mouth for her, saying "open wiiide."
If you feed the expressed milk by sing a started SNS on your finger intead of a bottle, this will help rid her of nipple confusion.
Try nursing lying down, on your sides (or you on your back, and she can crawl to the breast, like those newborns in the films. Sometimes if you let the baby take the initiative, they latch on perfectly), and let her come to the breast, in a darkened quiet room. Have your shirt and bra completely off most of the day, wear her in a sling. Never force. The falling asleep can be caused by over-stimulation.
Or, take a couple days completely off from bfing if it's all getting too tense. Use a slow flow bottle or Habermann feeder. Just have lots of enjoyable skin to skin. Take baths together, use massage.
Since she did it once, she can do it again!