FLEAS WONT GO AWAY!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Somebody please help. It’s been MONTHS of fleas on my cat and dog. Originally we took in a cat around October, we believe it had fleas. Ever since then my dog has been itching uncontrollably (flea dermatitis) . we went to the vet a couple months ago and spent $700 on 3 different shots, pills for my dog to take monthly and flea spray for the whole house. We followed his instructions with taking the pill cleaning the house regularly cleaning the carpets washing the sheets comforters anything cloth or fabric material. We gave the cat Advantage II treatment on his back, bought a flea comb, and bought diatomaceous earth and put it ALL over the house. It was everywhere meaning baseboards, all over the couch, carpet, rugs, EVERYWHERE. While the house was covered in the powered we separated the dog and cat from eachother at different houses. We let sit it for a couple days and vacuumed it up with a shop vac. A couple days later i used the flea comb and STILL found fleas on the cat. Meaning all of our work has gone to waste. (Washing everything multiple times and spending over$900 in vet bills) I don’t know what to do anymore. I’m frustrated. Someone with any advice it would be really appreciated.