Be a bit more patient with repetition, especially on rough drafts. Sketch out a form first, then put your theme in. Try putting in a B section or a variation of some sort, then come back to the first part again.
What are you trying to do with your song? If you're trying to tell a story, then you'll want to change the song as the story develops. If you're trying to paint a picture, don't vary it too much—it's okay to let it dwell on the same theme for a while. See the attached song for an example of a rather simple tune that paints the picture effectively and beautifully enough that it doesn't need very much variation at all. The song could be twice as long and add nothing more musically and it'd be okay with me.
You could also go into making movie scores. They tend to be made of many small songs, and that way you could also explore several different styles using a few overarcing main themes. Even if you don't do it for an actual movie, I think it's a great exercise to take a single theme and put it in as many different styles and moods as you can. It can really help in understanding just what it is you're building when you make a song. See the link for an example of...well, okay, not exactly what I'm talking about...but the idea is similar enough, and it's Victor Borge anyway.
Or hey, you could try writing a few verses to go along with your song. That way the focus isn't so much on specifically being musically interesting. Once you have the verses in place, you can play around with subtle stuff in the background if you feel like it, and that'll help you get used to developing a song meant from the start to be longer.
