I just thought of something.

What if Nagito tried to kill himself numerous times in the past, but his good luck always saved him in the end?

And, because every single one of his suicide plans failed, it led Nagito to believe he had to die this meaningful, important death or it just wouldn't work, which is why he's so fixated on 'dying for hope.'

The only time he succeeded in completing suicide was when he was trying to wipe out the Remnants of Despair, and even then, it wasn't a 'real' suicide.