So while I do fit the title question criteria, I don’t think my answer will help with your problem.
I recommend looking into the idea of having a fluid sense of self - basically a concept that there really is no such thing as you in a static sense, and that the thing called you can be dependent of internal and external fluctuations, so getting attached to one or multiple can be harmful. People who have a fluid sense of self tend to be more resiliant to things like conflicting narratives running through their head, because they are aware that they’re not actually tied to those narratives. I think it was originally a buddhist concept, so if they’re into stuff like meditation or anything new-agey, they shouldn’t have much trouble opening up to the idea.
It might not solve the trust issues, but maybe it will help them feel more comfortable in their own skin.