You have alot of issues - too many for a 14 year old tohave to face alone - hell too many for anyone to face alone!
But you need to start one step, one thing at a time as you can't do everything at once!! First you need to realize & accept!! That you are not responsible for your parents fighting (i have a 16 year old & yes my husband & I sometimes fight about things concerning her but its because we fight not because of anything she's done wrong, if that makes any sense?). Especially if your dad does pot, well he's to blame for that - not you!! & your mum chooses to stay & fight etc & do things that he disagrees with, her choice - not your fault!!
Second though is there another school you could go to, to get away from the maniac guy & also to make a fresh start???? I realize its hard to think about leaving friends but maybe if you changed some of your friends who are being hurt/threatened by the guy would change too. Also if he does kill you (or if your wrist cutting - even by accident - goes too far!) then you won't be seeing your friends!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
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Third, & here is the problem - at 14 I assume you don't have any income of your own that would let you buy any types of meds that might help your depression?? Like valerian, vitamin b's, sam-e, 5htp (tryptophan). Although maybe yu could get a hold of a yoga video/dvd & copy it - one with meditation included - this my daughter found & finds really helpful. Also if you can take a walk for 30 to 60 minutes at about dawn as at this hour something in the sunsrays affects the chemicals in our brains (while exercising) - even if overcast - to reduce depression & stress.
I think that's enough for now.