Dave, I agree that it's hard to separate the music from the lyrics. Those songs that "write themselves" usually seem to start with a good solid phrase, and the tone and content of that phrase defines the music. An acquaintance of mine used to bat those out at the rate of several per week, but only once in a while did he get a really good one.
On the other hand, some of what I consider my best stuff has come through extended trial and tribulation. (I believe it was Edison who something about 1% inspiration and 99% perspiration). I think it has to do with the intellectual level of the song. If the song has something to say, it may take weeks or months for me to figure just what the message should be. Then it may take even longer to capture the message in words that are suitably poetic and get the tone of the music and lyrics to mesh. I'm in the middle of one right now, where I can think of lots of good things to say, but fitting them into a consistent musical and lyrical framework is the trick. If I didn't already have a good verse and chorus that rhyme, I'd consider doing it with no rhyme at all.
I've done some other less meaningful songs that are still entertaining and fun to perform, and they were a lot easier to write.
One of the ways I approach lyrics (after I have a subject) is to sit down at the word processor and throw in every good phrase that comes to mind. Then, taking each of the ending words of the phrases, I list out all the words that rhyme. Every now and then, one of the rhyming words jumps out lays out the rest of the verse for you.
The ultimate reward is to be able to perform a meaningful piece of your creation and have it truly move an audience. It's a spiritual experience like nothing else. If you get there even once in your career, it makes it all worthwhile. I'll never forget it.