Ungratefulness

I have been looking after my granddaughters whilst living in a grannyflat out back. They are now 12 & 15 because their mother is an addict and a lousy parent.I seem to get abuse from all of them and have had enough. I want to sell my house that they live in and move interstate to get away from them.
I'm certain that the 15yr old is severely depressed and needs help which I've tried to get but at the end of the day I'm not her guardian so hit a brick wall at each turn. My 12yr old grand daughter was so close to my husband and me but now is abusive and angry all the time especially to me.
Both the girls know that their mum uses drugs. I just don't know what to do. I have had had 2 strokes up to now caused by stress and am frightened of another.
I feel my only choice is to walk away