I won’t post my text here, since it needs to be in your natural voice. But here’s my outline, in case it helps:

1. a clear “no” right away
2. gratitude, since I’m honored by my value implied in the ask
3. explanation that to stay focused on a bigger “yes”, I’m saying “no” to everything else
4. good wishes, and if my situation is temporary, an invitation to ask again next year

Four sentences is enough. Nobody wants verbosity he