my mum has some symptoms of depression [going to a psychologist to see what exactly she has], so i know what you mean. i'm sure my mum isn't nearly as bad as yours though, but i understand what you mean about the negatives. my mum is always angry, i'll do something nice for her [ie, clean the kitchen or something] and she'll harp on about how i haven't done a load of washing recently, instead of looking at what i've done. or i'll get home and the first thing she'll say to me is that i have work to do, my room needs to be cleaned, i need to do washing, etc, instead of even saying hello. she usually speaks really aggressively to the rest of the fam, especially if they say something that she doesn't agree with. so yeah, i remotely understand what you're going through. it sucks =[[.
...but yeah go to someone who knows what theyre talking about and can give you advice...it might be hard but seriously it helped me so much and I hope things improve for you, your family and your mum!
