I have seen several 'Self-Insert' stories similar to 'My Immortal' when I've been reading fan-fiction. Usually the same horribly-mangled approach to the canon, often with massively-mutilated characterisation all of which appear to be solely to allow the author to be the centre of events how they would like them to proceed as well as the lead character.
The thing is that these are usually very genuine and very earnest writers. Quite frequently, they are troubled young people - lonely, disaffected and disenfranchised. Writing S. I. fanfics is important to them. It is, in many ways, a kind of therapy for them. By imagining themselves as the centre and focus of their favourite fictional world (or, sometimes, a hideous amalgam of several fictional worlds - the basic concepts seen in the works of Anne Rice and other vamp-romance writers seem to be fused in quite regularly). By doing this, they are able to tell themselves that they are not outcasts or members of a fringe subculture. How could they be when Harry Potter (or whoever they are writing) is like them and their friend.
It's for this reason that I try to be gentle and not overdo criticism of such works. When a work of art becomes a serious support prop of the artist's psyche, then tearing it apart too completely threatens to also tear apart the fabric of someone's mental and emotional balance.
So, what is the truth about "My Immortal"? I suspect that Tara was a troubled teen (when are they not?). She wrote the story as a sort of emotional outburst, confronting her own feelings of alienation, her desire to be one of the popular girls and a slightly macabre personal fantasy world probably based on reading adult fiction that is unsuitable for an emotionally-vulnerable girl. It's quite possible that, after her self-harm/suicide attempt, she was put in therapy and, amongst other things, abandoning "My Immortal" was part of her treatment as it was both a codification and reinforcement of her unhealthy and self-harming behaviour and, through the increasingly aggressive feedback from trolls, a potential source emotional destabilisation.
Removal of the story and other social media sites associated with this phase of Tara's life is consistent with someone who has decided to close the door on her past and move on. If Tara ever now thinks of 'XXXbloodywrist666XXX' or other aspects of that part of her past, it is probably with embarrassment and regret.