try to latch her on without the nipple shield. if feeding time is stressful, make sure to do some skin to skin contact with her without feeding -- she may be associating the two and get frustrated. let her sense that it's a calming time and then slowly introduce the breast -- after a few times of her just nuzzling your bare skin.
nipple shields have their place, but oftentimes do more harm than good. sorry you're goig through this. just keep on trying and she'll get it
