Hi Adam,
I'd say you're putting too much pressure on yourself to 'create' a song in one go. These things evolve, grow from a small seed of an idea. I tend to write my lyrics first, basically like a poem, then sit down with the guitar later. Don't worry about if it's direct or any of that.
Whether its words first or music first it doesn't matter, like Alexander says, maybe strum a few chords and "la la la" along until you have a basic melody, then sing some words in place of the "la"'s , before you know it, you'll have that seed. With me, I tend to write a first line, sometimes plucked from the air, then I write a second line, maybe it rhymes, or maybe the one after that does..
Another good way in can be to sit down with a song in your head (a famous song, something you know and like) then write alternative words to fit the structure. Having this 'ghost melody' in your head can help you to get those words out. Once you have those lyrics, sit with your instrument and come up with a chord structure and melody, (disregard the original famous song melody) and hey presto, an original song
My last tip, and I've done this myself, is to write acrostically. That is, think of a phrase, something you want to write about, for example
'I want to write a song' then write it downwards on the page. Use each letter as the first letter of each line, before you know it, you'll have some lyrics..
In the dark
When all is quiet
All I can do
Now I'm all alone
Taken away
To another land
On my own
Words are gone
Really hard
In the dark
Time goes slow
Ever slow
And then it's there
Small idea
Opens up
Now I'm there
Got a song
hab..
