I think if you just let it flow,the song will just come out in it's own good time.It would be a mistake,if you ask me,to try and force things together.If you have enough reason to write a song,then you shouldn't struggle to put one and one together.
I don't write songs myself.Until my guitar playing improves enough so that my hands can 'say' what is in my heart,I won't write any.Now all I do is collect ideas.I carry a notebook with me( you get ideas in all sorts of places).
There's some articles I saw on a website called guitarnoise.You might want to check it out.Go to
www.guitarnoise.com
and click on the link 'songwriting'.
See what you think about it.