I failed to see what was not there

Good point Flash...one of the reasons I used 'an' was to convey the idea that at the time the subject (I) had no idea what was missing [being 'absent'] - (but did sense that whatever was missing was 'essential'), and that even the 'absence' was only acknowledged in retrospect. Does that make sense?
Besides, 'the essential something' would have sounded vaguely arrogant to me - if I had been the 'I' in that scenario, I suspect that my presence would not have not have constituted an essential missing element, even to myself :> Which is irrelevant I guess in the context of the poem, but then I'm big on irrelevance...thank you for your generous comments [not to imply that they were irrelevant in any way :> ]