I've discovered I can combine two songs with similar concepts into one longer suite. Then I don't have to worry that two songs are too similar to each other: they are sections of the same composition. They can have contrasting rhythms but be linked by being in the same key and by using some of the same harmonic devices.
Then a problem--all my songs written within month or two sound the same--turns into its own solution. I can also add on to songs written in the past.