I can relate to this. I don't have children myself, but my brother has a severe case of OCD and is resisting treatment. He is 35, unable to care for himself, and lives with my parents.
Whenever he's mad at them, he tries to get revenge by saying he won't get treatment. He is only hurting himself. I'm lucky because I don't have to live with him, but I honestly don't know how my parents do it.
I'm sorry for you parents who are struggling with mentally ill adult children.