I find that experience familiar especially regarding the wording of a line and I used to lament the loss until I realized I was fixating on the grains and missing the beach - you can't lose a really profound idea, if it were profound at all - you can lose the wording but that's just the packaging...something profound you'll remember - it's the utterance of those kinds of ideas that make a room go silent but unvoiced they still should light up the dark network inside you. Treat an apparent 'lost idea' as mulch to prepare for new growth.