Perhaps it's worthwhile to consider accepting a little help, in the music or in the lyrics, to get unstuck. There's the issue of whether co-writing credit is fair or expected, of course.

I heard an author state that Yip Harburg couldn't seem to come up with a last line for "Over the Rainbow," and that when that fact was mentioned to Ira Gerschwin, Gerschwin quickly came up with, "If pretty little bluebirds fly beyond the rainbow, why oh why can't I?" No co-writing credit was required.