I am not a mother of a teenager but I was caring for my son and two other teenagers when I was 19. Some things that you said in your post resemble disdain for this child . Yes, her age is 19 given her issues she is still a child. I really don't care for the statement Cutting herself, repeated panic attacks, and using guilt trips to gain the mother attention. those behaviors are not manipulation but serious issues only being able to escape in those ways. You have to look at her as a 14 yr old child with serious life threatening issues that she is scared out of her mind of. She's currently throwing a tantrum because of the changing family dynamic. This should be dealt with but should not effect your relationship.

I fully believe any persevered male conflict would send her into a panic attack and flash back to feel like you were like her step father. Yes, it is going to take more effort to get her stable on her own. She doesn't need discipline and her opinion on her mothers relationship should not matter. She needs structure, and intensive outpatient trauma care. Does she have a part time job or go to school part time? She will need a lot of extra care and support for at least the next 2 year to become an adult.