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My first thought on reading your post was what attracted you to this woman in the first place? My second was it was good of you to be a caregiver for your girlfriend during her illnesses, but at what expense?
We've had numerous cases of husbands/boyfriends leaving their partners upon a breast cancer diagnosis, but in almost all cases the guy was a me-first jerk. Not the case here. I don't have enough information about your girlfriend to say why she's so abusive and demanding....there could be lots of reasons...but nobody--man or woman--should just steamroll their partner...sick or not. If guilt is the only reason you're still in this relationship, you're not doing yourself or your girlfriend any favors. You're making yourself miserable and poor to boot. Resentment is the only logical conclusion.
What would I do? First, I'd talk to her. Tell her exactly what you told us here...lay your cards on the table and be sure to be clear about it. The camel's back broke already, and it's not likely that she'll be any different with this diagnosis than she was before. Second...talk to her family. Tell them the same things and let them know they will be in the driver's seat here...not you. You cannot afford it...either emotionally or financially. You do need to make it clear to them. Go armed with a list of phone numbers of organizations that can help them care for your girlfriend...the American Cancer Society, in-home healthcare and others. Contact the social worker at the hospital to get these numbers...they've got loads of information. Give a copy to your girlfriend too. Make any arrangements you feel are necessary...for your mental health first, your girlfriend second. An action plan can lift a load from your shoulders.
From my seat, David, I see you as someone who really does care, but you're torn between your needs and your girlfriend's. You HAVE to be able to live and eat...you need the job. Do what you have to to keep it. You're no chicken but you are being bullied...respect yourself too. If this seems overwhelming to you, you might want to seek counseling for yourself...it really DOES help, hon.
Time to get off the hamster wheel of guilt and obligation my friend. Take care of yourself...we certainly don't want to see YOU become ill.