18 and need some advice!

I turned 18 on June 26th. My family is moving to Alabama before the end of September. I was told I had an option to stay if I really wanted to. I decided to go but now I don't want to. I want to move in with my boyfriend who lives 25 minutes away in another town. I don't know how to tell my parents. If I just come out and say it my mom will try her hardest to keep me with her and might even cross some lines. How do you think I should tell them?