That

"THAT won't work for me."

and avoid DISEASE.

-or-

You can WORRY
for
hours, months or years.

Authority figures and charismatic people can have a strong influence over you.
People can ask for you to volunteer
to help their cause, beyond your capabilities or time.

They have the right to ask, to meet their needs.
You have the right to say no, to protect your needs. 

Instead of having excuses and reasons snaking around inside your mind, ready for a confrontation that doesn't happen, just cage all those snakes and use a magical word "THAT".

All you need to say is: "THAT won't work for me."
They are rarely concerned to know WHY.