I am looking for a software for writing poetry. Is there something
available that will provide this structure for, say, a sonnet, or
other forms? And then let you "fill in the blanks," so to speak? I
have tried to do this and with excel with limited/mixed results. Am I
alone in the universe in my desire for this product?

I remember learning that Thomas Hardy did something of the kind on
paper--sketched out the forms of a verse as blank lines and then
filled them in.  And Poe experimented with different forms of verse in
a similarly deliberate, arbitrary fashion.  Well, anybody who writes a
formal Shakespearean or Petrarchan sonnet isn't achieving it by
accident.

In software, it would have to be almost like a music-writing
application, wouldn't it?--because any verse that bases its line
structure in number of feet would have to count syllables and not
words.  And you can do that yourself.  All you need is the
specifications of the verse form, and there's your template.

I guess it seems to me that it is easy enough to count, say 5 iambs
per line in a specified rhyme scheme, say, ABBA ABBA CDECDE, and that
having it counted by software wouldn't help much.  The application
couldn't do the hard part.

Archae0pteryx