Lyric discussion by JewellyBug 

Cover art for Drown lyrics by Smashing Pumpkins, The

Lost zero, i think you're right. When he says "yesterday the sky was you" he means that yesterday (ie. before they broke up), his sky (ie. everything surrounding him, all he could see wherever he went) was her. That line has a lot of meaning for me, whether I misinterpreted it or not.