Always

I've got something different; some of which echoes what others have eluded to, but I'm going to twist it around.. after all, I'm interpreting this from a guy's perspective; a guy who has been dumped, and... god... maybe even felt some of what this girl felt, who has been dumped or left to watch her ex's new relationships unfold:

Boys will say 'yes' to anything! Love, sex, attention.. "with a boy like you your middle name is 'Always'. " The narrator--the old girlfriend who is still not even close to being over him--is saying to him, your yet-another new relationship is happening only because you can't say 'no'.

"I'd always love you." She's saying, 'don't you know I'm the real thing? I'm not like this new desperate poser.'

"You let the needle drop", means the dude let the latest new girlfriend weld her wily ways and got him to say 'yes', which I've already said is really easy with guys. By saying 'you let the needle drop', old girlfriend means, 'I think that you could do so much better... like choosing me.'

"Those conventions are taking their toll on you" 'Conventions' are more like conventions of the mind.. what this guy is conforming to.. the stuff "right before your eyes."

"The phantom is disguised. It's horrifying, right before your eyes. The phantom is inside" Old girlfriend: 'The new girl manipulated you and reeled you in, my old boyfriend.'

"The phantom is watching you." 'What's your next move, now that the this new manipulative girl has made her psychological move? I could be the phantom... watching you...'

This is a lament for the stupid boy she thinks she deserves, but will never have.