Possible burnout-- help?

Hi! I'm new to the forum but would appreciate any advice you could give. My father passed away last year and even though my mom is physically healthy, she is very dependent and cannot live alone, so I moved her up to live with me. She has had 2 breakdowns in her life, both years ago, and i was pleasantly surprised that she has held it together so far after his death. She hasn't worked since 1986 and my father retired in '88, so they spent all day, every day, together. I am a teacher so it was hard for her to adjust to me being gone all day but I've been home for the summer the past 2 months, which she's liked.

Here's the thing though, I'm having a hard time adjusting to living with her. She is constantly criticizing me, picking fights, developing mysterious ailments (especially when I make plans to go out with friends) and such. She wants my entire life to revolve around her and what she needs and I feel like I'm suffocating. I am an only child with no family of my own, so there is no one else to help. Plus, I go back to school in a couple of weeks, where I'll have 100+ 7th grade students also making demands of me. How can I survive all this?