My songwriting ability comes and goes. If I manage to write something, it'll be all at once and everything falls into place. If it doesn't, I'll end up kicking a couple good lines around for hours trying to find more to match them to, and eventually just give up. It seems that the older I get the less often it "just works", but that just might be my standards for what's good getting higher.
I pretty much consider myself like you; good with words, but not an artist with them. I just get lucky every once in awhile.