Not true!!! It is definitely not at all true that once a drug's effectiveness is gone "its gone". Everyone's biochemistry is different and for most people (sorry mammo), the drug will work again after a period of being off of it or when it is added as an adjunct to another medication. I was on Prozac for years, built up a tolerance, switched to Celexa for a couple of years and now am back on the Prozac and it's working fine. To boost it's effectiveness, my doc added Wellbutrin for a while. There are many combos that work better for people than just one single anti-depressant.
I have tried every anti-depressant out there, but the one that worked the first time (Prozac) still works the best for me (if I take breaks in between or use an adjunct med). Your psychiatrist is likely trying to give you something that s/he knows you did well on in the past because its a more sure bet than giving you a brand new one. That being said, tell your doc about your doubts and decide together what the best course of action is. Good luck to you!
Once the drug's effectiveness is gone, it's gone. Is your doctor a psychiatrist? They are mosy knowledgeable about which drugs are best for you. I got quite a few years out of Prozac but then it became useless and I had to switch to a different medication.