I am one of those people... when I'm asked to volunteer for something, whether it be retyping a worksheet or heading up the English department, I can't say NO! This is a problem for me! Sometimes my plate was so full and I was stretched so thin that I felt I was not coping. My inability to say NO forcefully and like I meant it, was my downfall.
I thought it was a sign of weakness, but it definitely is not! Sometimes it is good to say no and mean it. You don't need to be rude about it, but be firm. "Unfortunately, I will not be able to ..." "No, that will not be possible." Don't leave room for negotiations.
You also deserve down time, family time and me time.