Since you've been taking it since it came out then I suspect that your body has built up a tolerance to the drug and it's no longer effective for you. This is very common with these types of medications, and the first thing you need to do is speak with your prescribing doctor. When this happens they normally will switch you to another medication. NEVER abruptly stop this type of medication due to the withdrawal. Speak with your doctor about how you are feeling, he will give you directions on how to wean off the Prozac while starting the new medication. This happens to everyone with these types of meds, you know it's happening when things appear to be getting worse. But you've gotten many years out of the Prozac...time to move on to another. I hope this helps and best wishes.
For some people, like in my case, I am almost forbidden to take it in the spring and summer as it makes me manic. I am supposed to take it in the fall and winter, however. I wouldn't recommend stopping it all of a sudden as it may make the depression worse. But I am not a doctor so I would talk to one and start the "tapering" process in order to slowly come off of it. Then you may have to start again as the colder weather approaches. Good Luck, let me know how it is going.