When an adult sets boundaries with a child if then the child has a tantrum do you then give in? ...allowing the child to think "hhmm if I have a tantrum, then I get my way"..... noooo you then stay firm with your original decision, because that's what's best for you and ultimately her as well, since you're getting burned out.

The tantrum is another mode of manipulation, children learn this at a very young age, so I'm sure she has figured it out as well.

You can say to her something like- "I know you would like me to sit with you tomorrow, however I'm busy with other commitments"....then she has her tantrum....you then repeat what you already said, but give compassion also, such as- "I can see this has made you very upset, however as I said I'm busy tomorrow and will not be able to sit with you". You can tell her you'll call her instead? If you even WANT to, if not, then don't.

Sounds like you need some distance from her...she has you wrapped around her little finger. Trust me I have my own issues, so I'm not judging you, just trying to help you see what is so obvious to me looking in on your situation.