(((hugs)))
Do you have a friend with a different make of carseat that you could try out? Perhaps yours is uncomfortable for her in some way? Can you take it out of the car and see if she is OK sitting in it when you're at home? Another thought is whether one of those white/yellow/black baby book/freizes would distract her, or some music, or you singing? You could also try those distractions when she is in the carseat at home (not in the car), and gradually build up the time she spends there. It may sound obvious, but do check the seat for any bolts/lumps/bumps that might be uncomfortable, if you haven't done so already.