I'm 51 years old and again depressed. Any chance of it not coming back if I treat it? Do I need treatment for life?

Probably. My rule of thumb is a minimum of 1 year of treatment for the 1st episode. 3-5 years of treatment for a 2nd episode before considering getting off meds and if it recurs then ongoing treatment for life. Many people do very well with ongoing treatment.
This tells me that. This is at least your second episode. There is a 75% chance of another episode. Why risk another episode? The antidepressants that are available today are extremely safe. Would you stop your high blood pressure meds if you had good readings for a while?