Personally, I've always found it easier to put music to lyrics rather than the other way around. I usually get an idea built around a random phrase and expand on that - when I get some idea of how the song's going, then I'll have a rough idea of what music's going to go with it. I'll have a melody in mind when I'm writing lyrics - then more often than not, it's a case of working out what chords I need. Only rarely do I get a riff in my head first then put lyrics to it...just doesn't seem to happen that way for me.
Melody lines and chords - in fact, all the music to a given song - are usually far more rigidly structured than lyrical flow. Words can easily be changed to fit the flow of a melody - not so easy to change the melody to suit the lyrics!
Then again, this is just my particular style of writing - maybe it's something I've unconsciously adapted and evolved over the years to fit my own needs. Maybe that's what you should be doing - writing lyrics to capture the mood of your music? Songwriting's like any other musical discipline - it needs to be practised, to be thought about, to be honed. It needs to be worked at!
I've given - and been given - this piece of advice more than once. Carry a small notebook and your writing implement of choice. Write down phrases that come to you in everyday. If someone uses a phrase in casual conversation that strikes a chord with you, write it down. Keep all those random phrases and images somewhere safe - one day, if you're serious about writing lyrics, you'll be glad you did!
Vic