why cant i smoke cannabis in the uk legally?

I cant deal with life, i just give you a few points that ive been though, as a kid my mother was always trying to kill herself , i would come home from school find her at home after cutting her wrists, until 4 yrs ago she did manage to take her life by jumping from top of multi storie car park, my wife ran off with my best friend ,leaveing me to bring up my 3 now teenagers, been 8yrs now, if im honest i dont like people, i do have friends but i always pick holes in them once they have left, thats when i can be bothered to see them , ,i guess im classed as a voilent person as ive been to prison a few times for hurting people,4yrs for one offence ,ive seen shrinks an doctors ,they give me citalopram to help me, but they dont , one a day is what im ment to take, i take two, this turns me into a zombie an i cant be like that as nothink would ever get done, yet i can cope very easy when on cannabis, i can work deal with people an most of all be happy, i know theres lots of people out there been though lots more than i have . they dont need drugs or cannabis but i guess its how strong a person you really are, an i am week i guess, i take cannabis to get though the day not for fun , its the only thing that get me though my day, if i dont have it i snap so very easy, a quick eg is, i grabed the cashier by the neck at the local supermarket because he told me i couldnt have any more plastic bags to put my shopping in ,i was the lucky one that day as he gave me more bags with out any fuss,, had he of been rude or tried to push me of him i would of hurt him , i have been like this all my life, but since ive taken cannabis life is so much better, the pills i get from doctors either dont work or put me to sleep , cannabis is just right , not to high an no lows ,so why is it i cant buy or grow my own cannabis,im sure im not the only person like this,