I had a dream I was singing a sad song by myself but I was really good…
One thought is that singing in a dream shows you are generally very vocal about what you want and need from others when it comes to practical matters. Perhaps you are an excellent communicator with friends, family, your partner and at work and everyone knows where they stand with you because you are direct and honest.
If you were singing a sad song, then maybe you are feeling down about something at the moment and have recently told someone your true emotions or are on the cusp of telling them.
Singing generally makes people happy and uplifted, however if you were singing a sad song- it could be you wanting to let some deeper, negative feelings out because keeping them locked up is making things worse.
Maybe you feel alone in a more emotional matter but if you were good at singing in the dream, it could mean that you surprise yourself and are more resilient than you give yourself credit for. It might be an indication that you can get through this by yourself and can rely on your own strength.
Singing well in a dream can also indicate that you are good company for others and make those around you happy by just being yourself.
Source: The Complete A-Z Dictionary of Dreams by Ian Wallace