Your and your fianc e may have different agendas. She has to consider her 15-year old son and what would be in his best interests. Having been a mother at age 17 gives her a different perspective to consider. At age 25 you are considering your career. Then there is the agenda of the 15-year old.
Adolescents have an entirely different perspective on the world.
You are only 10 years older than he is, which in itself can create difficulties. It seems that when the three of you were away from the familiar surroundings and the support it offered, a great deal of strain was placed on the relationship. Teenagers are often very attached to their friends. Adjusting to a new environment is not easy. As a parent, your fianc e feels her son's dissatisfaction as well as her own.