I have been on the same snri for fifteen years and must stay on it for the rest of my life. will it eventually stop working?

Possibly not.: The risk of relapse is highest in the first five years the levels out gradually at about 40% after 5 years. But the risk of relapse is dependent on other factors including personal illness particularly endocrine illness, malignancy, even severe viral illnesses. keeping in regular followup with your prescribing physician is a good way to catch beginning relapse.