Why cant i keep my back straight up ?

Everytime i sit up straight it still feels like im slouching to the left and i noticed the other day my left shoulder is longer/bigger then my right and ive been getting back pains and shoulder pains (left shoulder) and idk if its because i have a one shoulder book bag its like a purse like this (pic example) http://d200fahol9mbkt.cloudfront.net/item/5782749/Tweed_Messenger.jpg and i always have it on my left shoulder because thats where it feels comtrable and its always filled ith heavy textbooks and stuff and when i dont have the book bag on i like still slouch to the side does anyone know why is this happening?