Advise

I've just turned 20 and have a mother who a lot of the time I can't bear she's an alcholic and also suffers from depression(she choose's not to take tablets and drink instead) I know I should feel sorry for her but I can't help being angry she's been like this since I was a young girl, I used to get embarased when friends came round. As well as that she goes absoulubly crazy at me at any moment for example when I was turning 17 my older brother got a friend to lend me some ID so I could go into town I ended up losing it altough i apoligied to the girl who's ID it was and gave her more than enough money to cover she kicked me out for 3 days. Another time she got so drunk she ended up walking out of the house and wondering off after I begged my dad to take me to go find her I walked into a pub I found had to drag her out at which point she urinated herself and fell to the floor in front of everyone. Today as it's mothers day I went in her room with a card from me and one from my brother(which I had to write because he's away) some flowers, an album and choclates , she was fine then went back to sleep until 2 at which point I went to see my friend for an hour who is a single mother then I recieved a text from my mum saying I'm a selfish bitch and got back to find her lying in the car with her dressing gown on passed out (my dad had took the keys) after banging on the front door she came in gave me some abuse told me to leave got dressed and wondered off. I have 3 older brothers ones a heroin addict so we're not in touch one lives in London and the other one lives with me, how ever he never seems to be on the brunt end of her verbal abuse just me and my dad. We've recently found out my dad has cancer and he's currently going through chemo, my dads the best person in my life and I don't know what I'd do without him but I'm sure he'll beat it, I've landed my dream job to go repping abroard but I feel to guilty leaving my dad with her I don't know what to do???