Thanks for the replies.
To begin with I tried listening to my "songs", and tried to think of lyrics that would suit them. The songs have their own moods based on my mood at the time (happy, sad, angry, laid back etc), so I tried to think of lyrics that would suit the music. But that didn't work out.
Then I tried composing a vocal tune for the chorus of each song, as I think it's really important that a song has a catchy chorus. But then of course that restricted the lyrical side even more, because there is a vocal tune there to fit in with too.
So now I think (like tone said) that the best approach is to forget about my music completely, and try to write lyrics on their own. I do believe that I have the expressive side in me, and that I am capable of writing good lyrics, but when I try to write them, I'm always trying to write lyrics that are more than just the typical simple ones you get. I used to listen to a huge amount of Pink Floyd, and I loved Roger Waters' lyrics (even though I never liked him much, was always a big Gilmour fan). His lyrics were always so descriptive, often using interesting words, and that's something I like a lot. So that's what led me to looking at online dictionaries, thesauruses, rhyming dictionaries, and random word generators. The idea being that I would keep producing a new random word until I found one I liked, and then I would form a sentence around that, and that might inspire me to add another line to that, and so on. That would eventually leave me with lots of lines of verse, of varying moods and situations, which I could group together into songs (which would be the hard part).
As for reading, unfortunately I don't do a lot of that any more. I do listen to radio shows and podcasts though, so I suppose audiobooks could be an option.
There have been a lot of difficult times in my life, so if I wanted to use real life as inspiration, then I'm worried that all my songs would end up being very depressing or angry, like Radiohead! Haven't said that, they aren't doing too badly...