Here's a method for forcing a song from lyrics. For your lyrics to be a song rather than poetry, you need a MELODY. This just means that you're singing instead of talking. Sing the words through. Now, this time, just hum instead of saying the words. Match what you're humming on the guitar. Write this into standard notation. You'll notice that your song is most likely in either a natural minor or major key. Then apply chords that fit the melody note. Try common progressions, and then reharmonize, substitute, or just break some rules and throw in some strange chord. Basically, take your vanilla progression you made and turn it into music. Make a riff that implies the chords. What you're doing is forcing yourself to try and make music out of what you have. When you don't feel like working on it anymore, stop, do something else, go to sleep. Your subconscious will handle it for you while you're not focusing on it, and you'll all of the sudden hit the perfect riff for the song. Good luck.