If this is what caused you to get the cat tested, then it is further along in the disease. Most of the sources I have checked out indicate at this point your cat will probably pass away in a few months, although you may get lucky and get more time. Frequent vet visits and possible intensive care at home may be required. If you've never put an animal to sleep, it is as tough as you think it would be. It is best to have a second 'type' of retest administered.
Details are in some of the links at page bottom.
The vet allowed us a final hour to cuddle and play with him. They take the animal away to give it a tranquilizer. If you have some final 'things to say' to your animal, now is the time. When he comes back to 'go to sleep', he will probably already be out of it. This is a powerful sedative which makes the animal more 'comatose' than asleep.