We are definitely in a V. Chatty (Darlings GF) throws temper tantrums when she perceives that Darling and I are getting "closer" then the two of them. I completely understand the description of the adult two year old. The biggest issue in our relationship is that she wants everything to be "fair", whatever that means. She wants Darling and my relationship to be exactly like their relationship. When it's not she has an emotional meltdown.
Darling has come to the decision that he is willing to let her go if she cannot accept our relationship the way it is. It is our relationship. He is tired of her controlling him and realizes that is what has been happening for years. He says he just didn't have a reason to change it until I entered the picture. He and I both hope that the DBT helps her.
We have separated our relationships more now. Although I still have some contact with her, it is much less. This helps so much.