Roger,
I'm obviously not qualified to offer anything other than my own thoughts, but to me it seems possible that Matthew Gentry always had some form of mental illness but it either wasn't noticeable or just ascribed to his personality. As for the trigger, again I'm not claiming any particular expertise, but what would seem reasonable or logical to us might strike someone with a mental illness as completely over the top, triggering an episode.
Bill Nash would be a much better guide for this then I could ever hope to be.
As for Lincoln embellishing things,
if you look at poem in its entirety, the first stanza is a bittersweet walk down memory lane while the third talks about the hazards one experienced living in the frontier and the excitement evident in a young boy's mind. Neither the first nor the third appear to me to be "embellished." While I have no expertise to judge the merits of poetry, it seems to me that something more than memories caused the second stanza to flash in Lincoln's mind as strongly as it did.
Best
Rob
Lincoln's Melancholy
