What makes a difference when deciding custody based on the best interest of the child" ?

My ex and I gave temp custody of our 15 year old to his mother when she was 3. Since she was 7 she lived with her dad. I never fought this due to threats from him. He has been in the Navy since she was 4. I have remained in her life constantly since the time she was 6, I have asked for her to come live with me many times, but was always denied by him. Once she got older I felt I would be ripping her away from school, friends, etc. Everytime he gets deployed he leaves her w her stepmother, her & daughter dont get along. 3 times already her dad told me to get ready cause he was sending her to live w me backed out every time. Father is deployed now, and stepmother fed up sent her on a plane to me. father knows about it but says he wants her back in october. I want her to be stable and not keep moving w millitary. I want custody. Are my chances favorable even though she has lived with him for so long? She is almost 16 and wants to live w me. What steps should I take?