“Is depression a lifelong illness?”

Many depressive episodes are reactive to some disappointment or loss. These are usually self limiting or can be treated to remission. other depressions are inherited through other disorders, Bipolar, Anxiety disorders, Schizoaffective disorder, etc. Look at family tree, have an evaluation by a psychiatrist.

Not really, it’s treatable now with a good psychiatrist and his proper choice of antidepressant for you. There are many and we're on the verge of selecting the antidepressants that fits the patient genetically. We know that psychotherapy is necessary because a psychiatrist can increase or decrease or change the antidepressant.

Unfortunately Major Depressive Disorder is a chronic illness that may last life long. However, good news is that it is a treatable illness and in the last three decades there have been tremendous research in this area and we have great tools of treating this chronic illness optimally to the point of full recovery and the daily functioning back to premorbid levels provided that patient's saty adherent to treatment. Particularly, if the first episode of depression is treated successfully with optimum treatment for adequate period of time, it may never occur again. therefore, early detection and optimum treatment are paramount.

Not always. The greater the biologic component, the more likely it will last, but often with episodic depression.

See this link.
https://www.selfgrowth.com/articles/what-are-the-causes-of-lifelong-depression