Why???? I agree with a few of the posts here. As a minor living at home, or even as an 18+ living at home, you are under obligation to abide by the rules of the home. I assume that you are baptized? I also assume, since you didn't deny it that your relationship with your boyfriend has become intimate? While I understand how you might feel about your mother, I ask you to stop and put yourself in her situation. Set aside for a moment the religious aspect and focus on the mother daughter aspect. As a parent, nothing is more frustrating that to have your child ignore what we think could be a harmful situation. Have you asked your mother why she feels the way she does? As the others that have posted have pointed out, what is wrong with you waiting until the circumstances are favorable and you are able to get married and thus avoid any situation that might put your mother in an uncomfortable position. Regardless of whether or not you agree at this point in time or not, when you did get baptized you basically made an agreement to follow the moral guidelines set out in the bible. Fornication is not just a moral guideline either, it is a sin in God's eyes. Even if you move away, the elders will follow through to be sure that your standing in the congregation is proper. They will send a letter of introduction, and if the elders feel that there may be a question as to your standing, they will include that.
Unfortunately for you, your mother appears to be looking at this situation in a human point of view. Afraid that the elders will find out that you behaved inappropriately with your boyfriend, rather than encouraging you not to disappoint God by your actions.