I have one alter (12) who is against therapy. She hates the idea of sharing, as her job is to protect us by not letting anyone else know details. I was able to get her on board with therapy by making a deal with her that I would leave her issues/stories be, and only talk about stuff that had to do with the rest of us. In return for me leaving her issues alone, she would let us talk about other things in therapy. She doesn't have to come out during therapy if she doesn't want to. It's worked fairly well, and she's learned that I am keeping my word (as much as I can).

After being in therapy for some time, I am now in the process of finding someone to work with who she might feel comfortable with opening up to, a little at a time. Again though, her stuff is hers and she can decide what will be said when. I just encourage her.

I don't know if that is something that might make your alter feel safer when it comes to therapy. One thing about having multiple parts coming to therapy is that it's kind of like getting a whole family into therapy. Everyone has to be okay with it at some level.

I would suggest that building a level of trust between you and the alters you know of can really do wonders when it comes to functioning. If an alter trusts you (even a bit) they would be more likely to put up with certain things that they normally would be against, knowing that you're not going to go back on your word, or whatever. Give and take between parts is much easier when there is a level of trust.