How can i exercise to lose weight when it hurts to move?

am a 41 year old male...i have degenerative disc disease...i have had neck surgery (discectomy)...i have a couple painful ruptured discs in my lower back that i will eventually need surgery for...i have arthritis in my neck and back, knees, and feet...many days it hurts so bad that i am unable to leave the house...this is very depressing because i used to be the picture of health...i was a bodybuilder, and always very active until about 6 years ago...my doctor wants me to lose about 40 pounds...i eat a very good diet, but when i try and exercise it hurts when i do it, and right now i am layed up with a "thrown out" back because i went for a bike ride this past weekend...i go swimming whenever i can, but that hurts also...how the heck can i get into a good exercise routine when everything hurts and i get "layed" up in intensive pain when i "over do it?"...it is depressing to be told by doctors that i am "obese" for being 40 pounds overweight and this is the reason for my arthritis and back problems...i feel they are being rude when they say this because i sure wasn't "overweight" before i started having pain...how can i get back into a good exercise routine again?...what can i do to feel better?...thanks in advance for any advice i get...