I wish there was a simple answer but people being what they are, there isn’t a step by step process for blending families that works for everyone. It may be that you are more ready to move on than your boyfriend so you are the one who is frustrated. It may also be that his relationship with his ex makes it more complicated for him.
From your description, it seems to me that your boyfriend’s problem isn’t really with the 14 year old. Just because she scared off one woman doesn’t mean that she will be successful in doing so with you. You are a different person. Hopefully, the teen has grown up some.
The problem of more concern is his relationship with his ex-wife. He seems to need more support in insisting that she keep to the divorce agreement regarding visitations and that she stay out of his personal life. You might be more successful in moving things forward if you addressed those issues directly. See if he can talk about what he needs from you in order to draw a clearer boundary around his personal life — including his involvement with you. He may not realize that the kids would do better if they had a predictable, stable schedule. Your relationship would do better too.
I wish you well.