I have a delema !

OK so I'm gonna make this as short as possible, I have a boyfriend love him do much he's good to me and all that jazz but an old friend/ex has always been close to me and I'm not willing to let him go but I don't wanna leave my boyfriend either and the ex and I planned on getting married when I'm of age but I don't wanna hurt anybody and I need to know the easiest way to let him down when that time comes that I have to leave , any ideas (I won't be 18 until next year)