My tale is filled with remorse & treason,
She betrayed her mother Rowena Ravenclaw by stealing her diadem..hence the 'treason'.. but Helena later regretted what she did..hence the 'remorse'
I rebuffed affections for a good reason.
Helena rebuffed Bloody Baron's affections who was insanely in love with her..and she did it for a good reason because Baron was a freaking psycho !.. who eventually killed Helena and then killed himself out of remorse..and both became the ghosts of their respective houses.
I did rebuff affections but I too was charmed,
She did rebuff Baron's affections however she couldn't resist Tom Riddle's gentle cunning charms even though she was always careful enough not to reveal her secret while she roamed the castle as a ghost.
A momentary weakness which caused a great deal of harm.
Yup! .. her momentary weakness did indeed cause a great deal of harm.. her most treasured family heirloom- her mother's diadem had to endure the worst kind of dark magic when it was turned into a horcrux by Tom after he cunningly acquired the diadem's whereabouts from Helena.
Its all long past now, for I am not even a part of the glorious history,
Which was all I yearned for , but what evaded me was victory.
Helena's treachery took place long time ago, right after Hogwarts was first established.. the only reason she betrayed her mother was that she too wanted to be known as an important person- a task at which she failed miserably ..
I am now left with grief which is eternal,
Yet nobody is bothered with it except me,may be because its all banal.
Her grief is indeed 'eternal' since she is now a ghost and will continue to be one for all eternity..she roams the castle in her grief , everybody sees her roaming the castle but nobody except her is bothered by her grief, since now the students and staff are used to the castle ghosts and hardly give them a second thought , it has become banal -old news sort of thing..
Nobody likes to walk in the shadows that are dark, a desire of mine which turned out to be a crime,
It was all that took me to commit the treachery when I was at my prime.
Helena was indeed tired of walking in her mother's shadows..and her mother's shadow was especially dark and hard to escape since she was very famous , respected and one of the co-founders of Hogwarts itself..its hard to carve one's identity in such dark shadow ( think Abhishek Bachan lol).. and yes, this uneasy feeling is what led Helena to steal her mother's diadem- 'the crime' ,when she was young- 'in her prime'..
What makes it all the more painful is the irony of my tale, 'the one I betrayed never betrayed me.'
Precisely!.. Helena did betray her mother but her mother never revealed Helena's treachery to anyone and pretended she still had the diadem..it could have been out of embarrassment but it is strongly implied that it was out of love for her daughter.. she even sent Baron after her daughter while she was on her deathbed because she wanted to see Helena at least for one last time..