First off, stop thinking about having or getting writer's block. It's a self-fulfilling prophecy. The more you think about it the more up-tight you get, etc etc etc...
If you're having trouble with something in particular, put it aside. There's no rule that says you have to finish a song just becuase you started it. Have the self-confidence to say "I will come up w/ another good idea", and put this one aside. Maybe some day you'll pick it up again, maybe you won't. Maybe some day you'll steal just one line from it to complete some other song. It's all good.
If you're having trouble coming up w/ anything at all then you need to kick the "editor" in you out of the room and let the "writer" do his job. Stop telling yourself every idea is dumb and instead just have ideas. Do some free-association writing as Fat_Satchel suggested, or write a story instead. Then strip it down to its basic elements and make it rhyme and ta-da: you have a song.
Literally every single thing around you is fair game for a song: the window sill, the door, the carpet, your pen, a smell. Pick something and muse on it.
And for cryin' out loud! Stop whining about writer's block!!!
(It'll just make it worse.
)
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