I am hoping someone has had a similar experience and could possibly help me with some insight.
Sorry for the book! 

I am a single Mother of a 19 year old son. I have dated on and off for over 13 years my now ex-boyfriend who has a 18 yr old daughter. The reason why I say on and off relationship is mainly off because of his daughter.
I have always been good to his daughter, taken her places, bought her gifts, phoned or messaged her for all her birthdays etc, When in her area picked her up for ice cream, spent a whole day with her at festivals etc, etc.
His daughter lives with her mother but visits every second weekend. Whilst we were living together his daughter started picking up jealousy traits which when I mentioned to him he would just shrug off. I came home one day to find she had taken everything that belonged to me from on top of a desk to inside of the draws and threw it all over the room floor, it wasn’t my place to say anything so I told her father and asked him to talk to her about it, all he said to her in a nice way was ‘please go pick that up’ and nothing more was done or said. Similar situations have happened on and off. I moved out at one stage one reason being our different views with children but since then we have tried to make it work again. My son and his daughter grew up together and sometimes they used to refer to each other as brother and sister, but then my son met a girl which she had issues with almost straight away and after four years of them dating still has, I have noticed that any girl that comes by whether friends or my sons friends girlfriends she had issues with.
We went on a family vacation together and I thought it may just be the turnaround for her, but I was so wrong. We had a wonderful time but as soon as we got back to her fathers house she became a different person. Each time I would go around to his house she would disappear to her room and stay there the whole time I was there. I spoke with him regarding it asking him to please have a word with her before it gets worse. Of course, he once again did nothing. Needless to say, it did get worse, she ended up packing her stuff and having her mother fetch her, a fight broke out between my boyfriend and his ex-wife blaming me and my son, she even told her daughter I was the reason they divorced, I only in fact met him 3 years after his divorce and the ex was the one that had an affair with my boyfriends best friend. His ex and his daughter have since black mouthed my son and I, called us names and told my boyfriend that my family talk about her all the time (this I all saw on his cell phone – he would only respond back to them nicely!). I have been so hurt over all of this, but what hurts the most is that he does absolutely nothing about it. I have tried so many times talking to him about it but the only answers I get are “what must I do?”, “what must I say”, “she will get over it”, “she is my daughter and I feel guilty for the years I’ve lost”, “understand that my daughter will always be protective over me” and the best one is “she’s just jealous”. We have since broken things off but stayed in contact, myself hoping things will get better? He has told me so many times that I am the only one for him and how he loves me but then when we try it’s the same old again and again with her.
She gets everything she wants. If she had to ask her Dad to jump he would asked “how high?”. He goes partying with her and her friends all the time, he hardly hangs around people his own age anymore, this I find extremely creepy, am I wrong? He says he is bonding? I don’t even see him as a father anymore, I see him as her friend, and I’m sure she is feeling the same. They talk about EVEYTHING! They discuss things about him and myself which I feel should be private, is this right? There is absolutely nothing he keeps from her.
Two weeks ago his daughter wanted to have a pre eighteenth birthday bash at his house. We were all invited for a braai (we were suddenly good enough??). During the evening (he is at this point still trying to win me back) he asked me if I wanted to go for a drink after the braai. When we had finished eating the teens decided they were going out for a while and his daughter said “Dad, you coming?”. While they were changing I asked if he was going with them, of course he replied! I tried talking to him about it again and told him I thought his relationship with her was unhealthy and I was not the only one thinking it. The teens around him and in our lives, my son, his friends, her friends etc have been saying the same. I did not want to even ask about what happened to his thought of the two of us going for a drink as he would have just changed it to me being jealous. He once again told me he is bonding with his daughter and his bonding is just different. Is this normal? Has anyone else experienced anything similar? I am really battling to understand? I have been hovering around for years hoping that this is the year he will realise what his doing, maybe next year? I think I have finally given up, but it hurts terribly when you think his the one and you have to give it all away for her selfishness. If he does not stand his ground now it will only get worse, wont it?? Am I wrong? As I said, I am a mother of a teen, and my son usually comes first as well, but there are limits and lines to be drawn, not? Is he not allowing her to control him? Is it healthy to hang out and party with your teenage daughter when there are adults around and you prefer to be with the teens?