I'd contend that his insecurities were about the culture that he found himself in... which incidentally is an important point.

From book 6:

    "... Ms. Maloney, as soon as you signed the Articles, you became a spacer." I almost smacked myself in the head for saying that. I couldn’t even imagine how many times I’d said I wasn’t a spacer. I sighed inwardly at my own stupidity.

I don't see it as him being insecure about himself as such. He's as much of a chad as any physically fit 19-year-old is...

He's still as culturally insecure at 20 as he is when he's 40 - trying to figure out how he fits in as a spacer who wasn't born to a spacer family.

So how does one handle the cultural insecurities at different points in one's life? At 20? At 40? nearly 50?

Book 5 gets a bit into the difference between the expectations of a planet (or station) life and that of a spacer and the stress there on relationships (and how they can work).

From the start of In Ashes Born this is stated quite clearly when talking to a therapist:

    My sessions with Mal Gains continued to be interesting even if I didn’t find them terribly fruitful. He introduced me to more family-of-origin stuff. He placed great store in the idea that I’d been raised in a culture of academe with its values of knowledge, truth-seeking, and reputation but then shifted to the culture of spacer with values like dependability, competence, and upward mobility.