"I can no longer (pioneer/serve as an MS) for personal reasons." Put it in writing and sign it. Hand it to the P.O.
They will want to talk with you about it to find out your "personal reasons." When they do, repeat after me:
"As I said, it's for personal reasons."
I would think that fading gets a whole lot easier when you get to the point where you assert a little independence and not feel that you have to answer their questions just because they ask them.
There is no boundaries within the congregation being a JW. Everyone is in everyone else's business and there is an unspoken expectation that you're supposed to answer any question posed to you. Nothing is too personal for them to feel that they can stick their noses in and provide a catch all magazine answer for.