To answer your questions, yes, some drugs lose effectiveness over time. Celexa has recently fallen out of favor due to a possible cardiac abnormality. Zoloft is very different and just may not be the correct drug for you, especially if you have yet to respond and if you are on at least 150mg. Prozac may be a very good option. Lexapro is most similar to Celexa but may not have the cardiac risk. Another try would be an SNRI, perhaps Cymbalta or Pristiq. WhaT YOU CANNOT IGNORE IS THAT SITUATIONAL FACTORS don't respond well to medication. If you have personal stressor, Medication plus counseling is a far superior option than either one alone. I would still consider changing off Zoloft if it has been no help - it should have lowered anxiety, improved sleep and improved depression if it is effective.
I took Prozac for about 10 years but have been off of it for almost 3 years. Do you think it would be effective again?