How do I tell my boss I need to switch to part time?

I accepted a full time Assistant Director position at my day care job a year ago. Since then, my health and family life has been affected negatively by it. We are severely understaffed, so if I go part time, or quit, I would feel bad about leaving. I could never do that to my boss and coworkers. I like this job and my boss and I and my coworkers are good friends. I’ve told her already that I’d like to be part time, but she said I’d have to give up the position and maybe take a pay cut. I am ok with that, but I think we can work something out where I still be her assistant and work part time. Since we are understaffed and there are multiple other stressors in my boss’ life right now, it really isn’t the best time to add this to it. I thought of asking the principal (my boss’ boss) if she has any advice. I really don’t see myself working anywhere else but here and I need a part time job and since my boss needs me, I want to be helpful. But it is at the expense of my health and family. Any advice for me?