Originally Posted by ctsketch
meaning Loki's scepter which is powered by the tesseract can penetrate the shield because you can't fight your own energy...
I suppose that is one interpretation. BW seems to take it to mean he's blaming himself for what has happened in that line (it can't fight you BUT you can't protect against your own selfish intentions/needs around it) right after that line she says "It's not your fault you didn't know what you were doing." and he replies "well actually I think I did" and goes on to explain about the scepter.