Harry is not actually the real Harry Potter.Voldemort, due to having split his soul so many times and with other dark rituals he performed on himself, cannot simply just die as anyone else can. After being hit with his own rebounded curse at the end of The Deathly Hallows, he awakens as Harry Potter on his eleventh birthday with no memory of who he was, but has most of Harry's memories. The entire world he is in, and everyone in it, are not real... he is in a hell of his own making by being forced to live as his worst enemy for eternity, though he will never know it. He is so badly damaged that he is forever trapped at the line between life and death, and this is why 'Harry' keeps looping over and over. Voldemort has indeed achieved immortality, but not in the way he wanted.
Harry will be free only when he gets it right.Reliving his life the same way? Not getting it right. He has to actually ensure the survival of every positive character from the books. Harry's main obstacle is giving up too easily. He gets attacked in an alley by a werewolf and then gives up. What he's supposed to do is live his life exactly the same way and go back to that alley until he beats the werewolf, no matter how many tries it takes him.
The final life where Harry seems to get it right but still fails is the actual series.We just read it from the perspective of Harry in self-denial.
This story is a metastory about Harry.This is what the world looks like to Harry Potter from within fanfiction: every time someone writes a fanfic, he is forced to live his life again. The lives in the stories are just the fanfics closest to the original, with the original Harry. The apparent constraining rules are the limitations from the actual series.
Harry will eventually exploit a loophole to escape, but it will cause his transformation into a dark lord.Since the loop resets every time he dies, regardless of the reason, he will eventually come to the conclusion the only way to avoid his dreaded fate is to eliminate death from the equation by becoming immortal like Voldemort wanted. All these repeats will ineviatably cause an irreversible damage to Harry's psyche and he will not hesitate to resort to an extensive research of the dark arts. He will use the infinite time he has now for a massive grinding of his skills and at some point he will even surpass the likes of Voldemort or Dumbledore, becoming an even bigger threat to the world than the former.
My Immortal is the timeline directly after the one where Harry gets it rightUpon finding that even getting it right won't do the job, Harry completely snaps from the horrific chain of events. In his insanity, he ends up committing a grisly murder, and drinking the blood of his victim. It is here that he discovers his love for the taste of human blood, and becomes Vampire Potter. He soon learns of a student named Enoby Dark'ness Dementia Raven Way, and decides that he wants to be friends with a girl who is just as insane as he is. He becomes goffik, and interrupts Enoby's breakfast to get her attention. The rest is history... At least for this timeline.