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.
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!