That's why you need to get a family therapist involved. Your ex-partner shouldn't have all the power here in the decision making about the mental health of your children while you two are separated. Can you just tell him, "We are doing family therapy for the health of this family. I'd like you to support me on this and give it a chance for the sake of our teenagers well being." I don't know how you can create new associations. That's something that a therapist can help you instill. If your ex-partner is adamant that family therapy isn't an option, then I think if you found a therapist for yourself during this family transition of separation, she/he will help you reconnect with your teenagers and create new associations for them so that they stop emotionally distancing themselves from you, if they have chosen sides already. You need to get your power back. And a therapist can help you do that.