Hello, I am Rafael. Thanks for asking your question - I'm here to support you. (Information posted here is not private or confidential but public).
I am very sorry too know about this very sad and frustating situation.
The story you have depicted here is very sad, sine it shows how you have worked very hard, being a really caring, understanding and supporting life partner for all this long, and played this parental role very well too, being compassionate, empathetic and very patient, regardless the challenges and painful circumstances his ex-wife has created, nor respecting boundaries and using their children to retaliate and undermine your life together.
Then when this should be a time in your life of further joy and fulfillment, because of your partner's decision to marry you, you are facing the fact that his daughter has chosen to take this very immature, selfish and insensitive attitude, not only not being supportive, but being uncaring and disrespectful towards you, which no matter how a person in your shoes approaches it, would always be painful and sad.
She is now already an adult, but very immature, what could explain why she is acting this way, once she has had the worst possible model and influence from her mother for all these years, and seems unwilling to face reality and acknowledge how unhealthy and abusive, neglectful and destructive her mother has been in their lives and in your relationship.
Unhappily children could get this distorted when parents do not play a healthy role in their lives. Now you need to set your priorities and assess the pros and cons of expecting from her more than what she seems willing to offer in your relationship. I do totally agree that denial, avoidance and repression around these very painful issues could not help any of you, even when projecting the illusion that everything is fine. I do support your plan that a family meeting, ideally facilitated by a marriage and family therapist would be the best approach too address these issues, but it would not happen, or would become counterproductive, as long as she shows unwillingness to acknowledge how unacceptable and wrong it it to retaliate against you this way because of her father choosing to marry you after all these years you have been togetehr, and everything you both have done for them too.
I think your partner should show empathy, understanding, respect and support towards because of how painful this situation has become, and that while nobody should push her to do something she does not really want to do, basic respect, maturity and boundaries are essential, and that's something the two of you need to work on, with professional therapeutic support, in that way you would better know how to address and cope with his daughter's position. Now she is an adult, and must hold full accountability for her own choices, feelings and actions, and that's how it should be approached by you.
Does it make sense?
Thank you so much for your kind reply to my email. It is good that a complete stranger can see my point of view. I know I am not perfect but I do feel that I have tried so hard with my partner's daughter that it really is time for her to try and understand my point of view now, and also the consequences of her actions.
My partner is fortunately coming round to thinking that we really do need to sit down and have a mature discussion, and if that does not work then your idea of employing professional therapeutic support might well help.
Thank you again for your help