If you want sad, try experimenting with different emotions related to sadness, like anger, or that feeling of hopelessness.
I recommend experimentation with suspended chprds, minor cgords, maj7 chords, and simple melodies. Also try using the major key sometimes but use a lot of 7 and 3 for a minor feel, like I did in Happy Song. Key changes are welcome as well
For some sad sounding songs to analyze I recommend Dirge for November by Opeth, Credence by Opeth, and Drive Home by Porcupine Tree. Also Death - Bite The Pain is a good one