Hello, I'm sorry to hear you are having such trouble. I have a 15 year old sister and she had the EXACT same problem when my mom was dating her now Hubby. My sister would tell everyone in the family that my mom spends more time with Jess (her Hubby) than with my sisters and that she hardly sees the kids anymore. This was totally not the case of course. My mom would see Jess about 2-3 times per week for a few hours after work. They always invited the girls to watch movies with them, or go to dinner with them etc. Well- the 13 year old loved that Jess was around and always wanted to hang out with him and my mom but the 15 year old refused! She told everyone she liked him and that she didn't want my mom to have to be alone in life after the girls got bigger and moved out but she still would not accept the idea. She tried to break them up and even succeeded in making my mom think she was doing something wrong by finding a wonderful man!! My mom went to a psychologist for advice and the lady told her that she could not just let this great guy go because she was having problems with my sister. She said to continue to be persistent in acknowledging that Jess was going to be there for a long time and wasn't going to let the attempts of a 15 year old deffer him. The psychologist said that children take is hard when a parent leaves and they do not want to allow themselves to get too used to anyone else for fear of being left again, and also that for some children, change is really hard to accept. The fear of the unknown, she said, allows them to behave in whatever way necessary to make things stay the same. Well, my mom and Jess kept going with their relationship and kept going towards marriage and though planning a wedding takes time they kept telling the girls all along that they were going to get married. My little sister's only concern was "what if Jess doesn't want us to paint our rooms?" which he replied to my mom "you tell those girls that this will be THEIR house too and they can paint their rooms and decorate them just like they were able to do before."
So anyway i'm sure you're wondering the outcome...well literally up until the day they got married the 15 year old still would be fine one second and the next lashing out. However, once they were married, and moved into the same house with Jess, she realized how much better it was that they were all under the same roof and she is getting more and more used to him being around all the time since she doesn't feel like she has to share my mom so much now. They go on family trips together and she has stopped lashing out so much now. She will still get upset with random things sometimes but they are all learning to adjust. Each day gets better as the Bridget (15) sees that Jess means good for my mom, yea, but that he also wants the best for them. He treats them as if they were his own, helped plan their recent birthday parties and helped decorate their rooms, and just acts like a dad would. My littlest sister was a baby when my dad left and really hasn't gotten to know him all that well so she's just eating it up and getting as much as she can of Jess, and Bridget is coming around more and more as she realizes how much more stable life is becoming with two adults in the house. It's been much easier for them to do the things they want to now that there are two people to help drive them to events, and now that my mom is not a single mom supporting 2 kids and trying to help a third go through college. My husband is in the Marines so we are now stationed quite far but from talking to my sisters and mom and Jess and other family and friends...everything is going to be just fine. I would say to you just be persistent as you said. Your daughter will adapt as kids do, but just realize that even though her fears and behavior may be childish, she is a child and just continue to treat her concerns knowing that they are REAL and that however irrelevant they may seem, they are still important and for some reason she is still feeling that way. Just don't let go of something wonderful because your daughter is acting out though. Continue to talk to her and be open with her on the progression of the relationship and if it comes down to marriage, include her in the planning and such. Just show her that she is still very important to you (as I'm sure you've already been doing). As said by another member, continue to let your daughter know that if she has a concern you will all deal with it as a family but that she can not continue to act badly without trying to find a solution or else there will be punishments for her behavior. My mother stuck to this also to show my sister that it was not ok to make Jess feel unwelcomed when he was only being nice to them. Good luck and I hope this gave you some encouragement :)