years of frustration so sadly resulting in violence

Hello SANE folk, I am beside myself because in a very heated argument after 4 years of emotional abuse, I just hit my just -turned -18 year old son. I can't believe I did it and can't see any point going on now, as he will never forgive me. I also pushed him once before about four years ago when he was refusing to stop punching holes in the wall over anger about I didn't know what, and its been four years of silent treatment since then (he doesn't talk to me, has only whispered for four years) school refusal, weekly punching up or kicking in the house (22 holes I have to gyprock repair,) many rules ie I am not allowed to look at him ever, be in the same room as him ever, he has punched me when I woke him up per his request, if I go to the toilet in the night he screams at me that I am disturbing him, he has woken me up to tell me that he will slice my throat - he has sprayed hair spray in my face and tipped juice on my head on the rare occasions when I have been allowed to drive him somewhere (at speed, on the freeway) there are weekly if not daily threats to take his own life with long calls to the police to track him, him often not coming home until late and then screaming at me, heartbreaking refusal to let me in, let me apologise. I am a single parent of three, working full time to support every wish of these children. I take anti-depressants which I stopped taking as they were interfering with another medication, and have mainly 'sucked up' this abuse from morning until night saying' Yes, you're right,' and 'Yes, I'm sorry.' (I think he is angry with his dad, who he hasnt seen in 5 years, who left out of the blue saying he didn't like that I wore odd socks and had found someone else....for it then to be found out that he had had 6-7 relationships throughout our 17 year marriage.) On this day 4 days ago when I lost it and hit my son, it was in response to my son having said that I was not to see him at all on his birthday as he knew I would 'be myself', and this would force him to commit suicide (I didn't and don't know what he meant about 'being myself.'.)So whilst his brother and sister gave him presents I sat in a car park for two hours. When I returned I put some presents and a cake near him. In the middle of the night he woke me up screaming that I had ruined his birthday because I had given him food as a present. (He had not explained this rule before, and the one act of family love he had practised was buying us interesting little food items, so I thought it would be ok.) He screamed that he had thrown everything in the river. I had chosen these things with love and care and had just had enough of the unfairness. My adult daughter has left the house on this issue, herself aghast and very judgemental that I would hit him. (I am also coping with menhorragia, meaning I am being treated for anemia- but the treatment has had me menstruating for 45 days , very draining, scary and I don't know when it will stop, if at all.) I keep thinking of the medications I have in my glovebox and wondering how many of them I need to be at peace. No-one will ever forgive me on this - even though he has practised so much violence on me- and can not see how pushed I have been. I know I shouldn't have hit him, but I did, and have tried to offer apologies but they will not hear them. (He never apologises to me for what he does to me - once, I crashed the car in response to him spraying me in the face with hair spray and he just got out of the car and went to his drama class.) Yet they continue to live in my house and have me cart wood, prepare all meals, provide electricity, computers, internet. There seems to be only one way out of this.