Lyric discussion by Pen1955 

Cover art for My Manic and I lyrics by Laura Marling

As I see it, this is an internal personality conflict in which the split mind of she and he become one, and thus have won, by calming down.

This is very similar to how I also see it; the "manic" actually being a persona of herself. This interpretation could also be supported through her progressively saying throughout the song, "There are the reasons I think that...You're ill", then "We're ill", then "I'm ill", as if she's possibly accepting her of "mania", if you will. Then again, I could be completely wrong, but it's an interesting interpretation!