As sad as you are, at least you're singing about it. It could be much worse. Unless you're Nick Drake, most states of depression render the affected mute and without energy.
Singing is a positive exercise. It's done best with a smile. This notion makes the phrase "Singing the Blues" close to an oxymoron.
Pick yourself up. Sing. Dance. Create something. Use whatever is sucking your happiness out of you as leverage to create value. Then just watch and see how quickly your sadness dissipates.