20 years old, still have severe acne on back?

[G]

through highschool i had acne all over, mostly on my forehead, neck, chin, chest, shoulders, back, etc. eventually it all slowly dissapeared. now i no longer have acne anywhere EXCEPT for my back. and on my back, it has gotten much worse, and i would classify it as severe. it is uncomfortable sitting in chairs or laying down a lot of the time. i get such large zits that they have to be popped, or even pop on their own, and when they do it is disgusting. they are like giant bubbles of blood and puss. i really dont know why this happens to me. i dont use any acne medication mostly because i cant afford it, but also its so damn hard to clean your own back...if anybody has seen a similar situation or has any ideas of what i should do it would be nice. ive been patiently waiting for this to dissapear like all my other acne did but im getting the feeling thats just not gonna happen.