Hi, so sorry to hear about your little girl, not just a cat but your companion over the last 15yrs, I have had to have several animals over the years put to sleep, and one with the same thing your little girl has, well, now's the time to let her go, don't do what some would by giving her flushes tablets etc, because at her age it really wont help, only make her more uncomfortable, I am so sorry.
They ask if you want to be there with them for the first relaxant shot, or be with them minutes later to help them slip into deep sleep and no more pain, yes it is upsetting but it has to be done not only for her sake but yours too, let her go, stay as long as you like they will let you stay till you have said all your farewells, I will be thinking of you, oh and one last thing, thing long and hard about bringing her home with you or leaving her there, I left one of mine there for them to takecare of, and do I regret it, thinking of you, L x