Woah, I thought she said "I'm diving again" (which I thought made sense, since she follows with "I'm going under, drowning in you) not "I'm dying again".
I think this song is about her wanting to get away from a bad/negative influence but the force is too strong for her to ignore. The bad influence could be an abusive or unrequited love, or perhaps something that has nothing to do with a relationship whatsoever. Man, they're beyond words. =P
But anyway, awesome song. "Going Under", "Whisper", "My Immortal", "Like You", "Taking Over Me", "Tourniquet", "Bring Me to Life", and "My Last Breath" are my favorite songs by Evanescence. They're great. =)