"I'm sorry I hurt you. I tried not-caring out of a selfish (though normal) desire not to feel pain when you didn't seem to appreciate or care about me. I have learned three things; 1) I still DO care, because knowing that I have harmed you like this makes me want to throw up, 2) It's not your job to appreciate me. I'm the adult: it's my job to care for you and support your mom in caring for you. 3) We can't selectively numb our feelings - I was trying to numb the hard moments of parenting, but there's no way to do that without numbing everything, which made me care-less in doing that session in a way that hurt you. And maybe an immature part of me was trying to hurt you just to see if you really didn't care."
PS: A licensed therapist should be helping to insure that your session is confidential and not being recorded.