OP, I think this is something my DH can relate to. Although he hasn't been falsely accused of anything but I doubt he will get over the fact contact was suddenly stopped sometime after separation from his first wife and he never sees the children who called him daddy for 10 years. He has a box full of photographs, letters and father's day cards that the children gave him and periodically he likes to go through them and remember the good times.
He learnt the hard way that if you choose to be an equal parent as a step parent without any of the legal rights you basically end up losing your family and there is nothing that can be done. He too was put into that position by his ex, not knowing any better he went along with it.
I think it also confuses the children, they now have another father figure in their life and another name change.
Your DSC may have divided loyalties now? Unsure how to react if they were to see you out and about and sometimes I think for teenagers it is easier to push people away you love and rebel. She might well miss you a lot, but might feel abandoned and left with her parents who were not there for her as much? Perhaps she is trying to ruin your relationship with your son because she wants him to suffer like she is?
I am just guessing though.
DH holds onto the belief that one day when his stepkids (exDSC's is that the right term?) Are adults they might want to get in touch.
I wish you all the best of luck and it took DH a long time to come to terms with it, and its not something you can easily just get over like some may think. Xx