Often--and this even happens in professionally written and directed storylines--the flaw is external to the character (like the dead wife, but also some sort of victimization that was forced onto the protagonist--abuse, trauma, etc.). Everyone who has lost a loved one or been abused or suffered a catastrophe doesn't act the same. Their personal and unique response reveals the flaw.

In a tv series I've been following (Russell Lewis's "Grace"--some spoilers below), the protagonist's wife has disappeared. The uncertainty has left him reluctant to commit to a relationship, but at the end of the second season, he has decided to take the plunge with a new relationship. His flaw has been fear resulting from uncertainty, and he has (with much anguish and many setbacks) overcome that flaw. Then at the season's close, he receives word that someone has discovered evidence that his wife is alive. This twist ratchets up the flaw: now he has to fear betraying his current lover or betraying his possibly still alive wife.

What I'm learning from your post and from the series "Grace" is that the most effective character flaw isn't an initial glitch (an imperfection or annoyance from the past that crops up from time to time and creates occasional problems), but an integral part of the protagonist that develops (and grows) over time as the story develops. It's the proverbial snowball rolling down a hill, until the final crisis and resolution. The flaw of the protagonist of Grace was in place even before his wife disappeared. BTW, this sort of dynamic is even more evident in Lewis's masterpiece tv drama, "Endeavour."