You are clearly being exploited. You are under no obligation to take on extra shifts, and you should be paid for any extra hours you work. Insist on that - and resign if she refuses.
You do NOT have to cover. You can say no. As you never say no then naturally your boss asks you first because she is not a mind reader and thinks you are happy to do the extra hours.
Say NO. End of story.
I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.