Lyric discussion by FabTemp 

Cover art for Sundown lyrics by Gordon Lightfoot

Combined with Jeff's comment about the Crawdaddy 1975 quote, I'd say that these lyrics are about a woman he loves being unfaithful to him or his fearing that she's being unfaithful.

I love the alcoholism allegory explanation, but I don't believe that's what's going on in this song. Although, being in love with an unfaithful person is very much like being "addicted" to something.