How do i tell my mom that im moving out on my 18 birthday?

me and my boyfriend has been in a relationship for 1 year now. on august 1,2009 he’s moving to minnesota for college. my boyfriend wants me to move in with him when i turn 18 on november 4, 2009. i said yes. i’ve been telling my mom that i’m moving out on my b-day but she doesnt believe me. how can i let her know that i’m serious and that i’m moving out on my 18 b-day?