Scoot, I'm sorry you're going through this struggle with your kids. It's a painful path you find yourself on. There is a very good article at the bottom of my post on detachment which may be helpful for you. There is not much, if anything, we can do to change the behavior of our adult children so generally, the best option for us parents is to start detaching from their behaviors and to learn a degree of acceptance for the things we cannot change.
*It may be a good idea for you to seek professional help, a therapist, a parent group or support group of some kind, if your kids have mental issues you may find NAMI (National Alliance on Mental Illness) helpful, you can access them online, they have chapters everywhere. There is support out there for YOU, you may need to dig around a bit, but you will find it. It's important to begin to take our focus off of our kids and put it back on us so we can gain the strength necessary to move ahead in our own lives and make healthy choices where our kids are concerned.
Many parents here have experienced their kids posting negative, untrue and hurtful comments on Facebook or other social media or simply to other people. It seems like a somewhat common ploy these difficult kids use to make themselves look good, make the parent feel guilt so they will cave and give in to the kids demands, pump up their own beliefs or experiences so others will think a certain way about them...............all kinds of negative reasoning which hurts parents. You may want to consider blocking her so you are not subjected to it.
The best advice I can offer you is to get support for yourself, start putting yourself first and taking care of YOU, do nurturing kind things for yourself and find ways to express your hurts, angers and feelings. This is tough, we are not trained in this kind of parenting, which is why I always suggest therapy or some form of professional support, it's difficult to detach from our kids on our own, we usually need help. I'm sorry you are struggling........I wish you peace..........