I think he's feeling guilt over his inaction due to not comprehending what was going on at the time. She could've been collapsed for a long time without somebody's help, but he misunderstood as her sleeping.
Overwork can lead to a plethora of things IIRC, one of which can be a heart attack, for example. If not helped in a timely manner, a collapsed person could either live while suffering from complications or die.
In short, IMO he's blaming himself for his inaction possibly leading to his mom's suffering and eventual death.
Goold 'ol 'woulda-shoulda-coulda'.
'If only I knew there was something wrong, I could've called for an ambulance much earlier, and everything would be fine' kinda guilt.