The heart of a song should be a simple idea, a single experience, or a universal emotion. It's a song; don't try to make it a novel.
If you find that you need the entire song to lay out your message, then you're already lost. You should give up on the song and write an essay, a blog post, or perhaps a poem.
Then, after you've gotten that out of your system, come back to what prompted to you to try to write a song, and see if there's a simple idea hiding inside your tangle of complexity. There usually is, and it's worth taking the time to get it sorted out. After all, that is your job as an artist: to make things simple — to make it possible for your audience to grasp something that might otherwise elude them.
It doesn't take any skill to create complexity and confusion. But it takes everything you have as an artist and songwriter to make things simple and clear.