Word Request – What Do You Call an Episode Not Closely Related to the Main Plot?

I'm not sure if it's the 'main' part (after all, the setup is just as important) but you're probably looking for the phrase punch line (also spelled as a single word punchline):

the sentence, statement, or phrase (as in a joke) that makes the point

_{(source: Merriam-Webster)}

It's often used for jokes which are like short stories; I'm not entirely sure if it applies to Q&A jokes as well (I'm not a native speaker).

Come and sit. I want to tell you the story of my life.

Though the story of my life is often used to describe something bad that happened multiple times in your life, e.g.

I asked the new girl at work to go out for a meal - and she got off with the waiter. That's the story of my life!

(source: Wiktionary)

it can also be used in the context you describe, cf. this article:

But it's not stupid at all. Though perhaps the facts of someone’s life, presented end to end, wouldn't much resemble a narrative to the outside observer, the way people choose to tell the stories of their lives, to others and—crucially—to themselves, almost always does have a narrative arc.

The story you want to tell will no doubt have 'something bad' that happened multiple times (probably many of them) so I feel using this particular idiom is warranted here.

Alternatively, you can use the phrase you already mentioned, life story:

Come and sit. I want to tell you my life story.

The Free Dictionary defines it as:

an account of the series of events making up a person's life

but this does not carry a negative meaning per sé.