I've always understood that Bond suffered from a long illness for which he'd been treating himself with narcotics, presumably morphine-based. He had a nurse, didn't he, who left the room where he was lying for a minute, and he then upped and threw himself out of the third-floor window? He'd been bed-ridden for weeks.
It stated in one of the press reports that he was awaiting an operation but had long despaired of his health. If he was anxious about a possible operation, couldn't sleep, his medical practice was suffering and he may have been suffering from a long term addiction to narcotics due to a painful illness, he might well have impulsively decided to end it all, IMO.