I read somewhere, that Shane Told meant two different things with My Heroine. The drugs, but the female hero too.
The Female, could've saved the man's life it's about. But than se betrayed him or something, and he got depressed. But now he wants to get over it, and be happy again.
And the drug version, I Think. Is about it helped him forget things, and than he became an addict, and that he couldn't live without it. And now (because the: I will save myself) that he wants to come clean, just what Nekomatarashi said.
I don't think people should argue about what this song means, because I think, that it means what you want it to mean.
Besides this, I still love this song, and it's my fav.