I would not say anything about her step-father. That information really isn't something that needs to be shared with her. That is between you and him, and children (even adult children) should really be left out of it. It's also not the child's job to protect or feel sorry for the parent. It can be very uncomfortable when (adult) kids are put in the middle and it can cause them to become distant-- which it sounds like it is in your daughter's case. While I don't know anything about your situation other that what you have posted here, I can say as the daughter of a mother with mental illness, it can extremely difficult for the child and, sometimes, distance really is needed. Parents often don't recognize how much children really are aware of and affected by growing up around mental illness. Your daughter is barely an adult, and she may need a break to only have herself to worry about right now. It really isn't her job to be a caretaker or support to her parents at this age. She probably needs to spread her wings and figure out who she is. It doesn't mean she is selfish or uncaring. She might just only have the energy to take care of herself right now.

In the letter, it might push her away more if you lost all of the things you are upset about and all the things you think she is doing wrong. Have you thought about what positive things you can bring to her life or what positive things you can say about her? Kids need to feel loved and supported, not just criticized. Otherwise they will pull away.