Advice on telling my 12 year old step-daughter I adopted her when she was 3.

I adopted my (soon to be) ex-wife’s daughter when she was three years old. Her birth father was never really part of her life. He probably saw her for a total of 20-30 hours during stop and start stints of supervised visits in the first 9 months of her life. She has no memory of him and has always referred to me as dad. At this point, she is not aware that I am not her biological father. My ex and I had 4 more children before we separated two and a half years ago. Ultimately, I am beside myself with guilt over the situation. I attempted to be upfront with my ex about my view that my daughter should be informed as early as possible about the adoption. Ultimately, I always deferred to her desire to ignore difficult situations (it was a running theme). I acquiesced because I felt then and still do to a point that it is “her news.” I mean, I chose to adopt my daughter. I chose her. Anyway, before I get too far off path, during a recent doctor appointment, the mental health professional (outside of our daughter’s ) rather strenuously reinforced my view to my ex that it was well beyond the appropriate time to inform my daughter of the truth. That if we waited any longer that it would be used against us as outright deceit — a behavior we’re struggling with her currently. This seemed to finally get my ex’s attention. Sooo, the suggestion from her therapist has been to tell her together as a parental unit, but also to write her letters individually. I like this idea because it allows her to have something to read after the initial shock. I fear that she will shut down and not hear anything I say beyond the initial sentence. She has stated that I will not be able to read her letter to my daughter, which I don’t think Is necessarily fair. My additional fear is that somehow I will be forced to do all of the talking, as my ex has essentially sat mute and made me do all of the talking when informing the children of anything involving the divorce. My ex has also suggested that she wants her parents to write a letter to be included with our letters. Her rationale is that her parents were such a huge part of her life before she and I got married that “it only makes sense.” I worry that this will be yet another way to attempt to manipulate my daughter against me in what has been a horrible 2 and a half year long custody battle. I’m ashamed I let it go on this long. I’m terrified that my daughter is going to turn against me on her own. She is a confused preteen who is angry without even knowing any of this. Add in the potential manipulation of living primarily with her mother and maternal grandparents who most likely wish me dead. Any advice at all would be greatly appreciated. I love my daughter so much, but I understand that I may have to let her go and let her come back to me on her own and in her own time, and that scares me to death.