Do you sometimes plan the structure of a song in advance? I'm torn between free-form-writing as it comes and the clarity that comes with a fixed form. (And also it's much easier to work towards "let's do an AABA tune".)
it's mostly impossible to know exactly what the form will need until you start writing it, however, the song will offer you clues. sometimes the initial idea will very clearly be a 'chorus', and sometimes you get a clear picture of what it will need. other times, the first idea will be like the title at the beginning of the A of AABA. it's really all about taking the initial idea and 'diagnosing it', with the most important question simply being 'is this the beginning or not the beginning'