Honestly I don't think you need to go into the details. But, there is the chance they'll ask ... I'm just throwing things out there I guess. I didn't give too much detail in my letter, just basic facts.
Therapists don't start with any pretense. Any therapist that does, would be unethical. They come in and assess the situation, talk to you and difficult child, they decide what they can to do help amd what sort of treatment/services they can provide. I know you feel betrayed by the system, but any therapist would be there for you and your son. They would report back to FAPT or whomever, but that doesnt' mean they have any other agenda other than to HELP you. That also doesn't mean they betray confidentialities other than basic information about progress. Again, ethics.
