「大切なのは、自分のしたいことを自分で知ってるってことだよ。」

Have come across this quotation and am trying to figure it out. The first part seems straightforward:

大切なのは、
taisetsu na no ha
The important thing

The next two parts are more difficult for me. We end with a "da yo", so the basic structure seems to be the usual "A ha ... B desu". There's a parallel "to...(by) oneself / on one's own" (自分で). "自分で知っているって事" is throwing me: knowing on one's own, but then that's quoted (or "what's called / the thing spoken of as"—what's spoken of / referred to as knowing on one's own? "自分のしたいこと" should be "the things one wants to do", and the を means it's the direct object of...what? Must be 知る—but then why the いる form? And how much of the thought does って cover? Maybe I'm overinterpreting the 知っている. Maybe it boils down to "the important thing is for a person to know what s/he wants to do". But that's not satisfactory for ってこと. What's called knowing on one's own? The important thing is the thing called knowing on one's own what one wants to do? 難しいなぁ。

Maybe the って doesn't need to be so literal. There are about 22 million instances, per Google, of "って事だよ", and 44 million of "ってことだよ". Plenty in romaji, too, plus misc "tte" that don't make a lot of sense to me.

Googling tells me it's a saying from the Mu-Min Valley. The source seems to be something called Snufkin (スナフキン). It all seems to be from a book and Finnish. So I've been looking around for likely quotations from Snufkin and the Moomin Valley, which is a pretty good sign that I've done as much thinking about this as I'm going to.