Personally I would just tell them that you have thought long & hard about this move & have decided it is the best move for you to make. (Period!!!!) No need to say anything more. By their age, being independent is expected & it is their problem if they aren't. We all make choices for our lives, good or bad & we ALL have to live with the consequences. It is time for you to take care of yourself & let them deal with their "failure to launch". They will either choose to launch or fail miserably but that is their choice, not yours. My daughter struggles at times & even at her age, I help if I can but she is independent & works hard, so mom's help is truly only when she really needs it.
One thought about them visiting.....give it time before that is ok or you may end up with at least one who thinks they can just move in when they visit.