I cannot sit up straight in a chair without pain?

I have never really sat properly in a chair i have been fairly inactive in my life i was obese so i sat in front of a computer for hours upon hours and i sat bad slouched and with no support for the back so i lost all of my weight and now im trying to sit upright in a proper computer chair but i physically cannot sit up its so painfull my entire back starts burning and feels like its on fire and it feel slike im injuring myself sitting up. Its been like this for months and i cannot sit upright in ANY chair. Any tips or ideas?