I have had a bash at this songwriting lark. I have no formal training in composition. I believe the same was true of Paul MacCartney and several of the classic show song and jazz songwriters.
Speaking only for myself, I tend to find that my better efforts seem to have a life of their own. I usually "hear" them before I write them down, so the words and the melody usually come together. There is nothing particularly "creative" about it. If I can't hear a song, I can't write it down! If I can't sing the first line - or main line - of a song straight away, I am rarely interested in trying to make anything happen.
I think you get rotten melodies and lyrics from trying to force words and tunes together. To me it feels like an arranged marriage, whereas I am keener on love matches!
This is not intended as a shot at those who prefer to use melodies, which they already admire. Arguably, that is a more creative process, because it is bringing something into existence, which was not there already!