JW Freak,
Congrats. You're not a coward. Your reasons for resigning a voluntary postion should not be of concern to anyone. So, don't feel bad about giving them the stress/depression reason. It's probably true anyway. I know it is for me and I've been playing that card for several months now to set the stage (probably will be doing the same thing next month).
Of course, the sec will be call the PO who will in turn set up a meeting with you & the entire BOE. Stay the course with the stress depression reason. I'm sure most of them will be able to relate. Tell them you can't be of encouragement to the friends the way a good shepherd should because of the depression and therefore its the "right" thing to do to step down.
I really think if you go out the nonconfrontational way, they won't pressure you as much.