I used to be sad all the time. Not that kind of valid sad due to exterior events, but the sort where you are earth shatteringly, mind movingly sad. The kind of sad that would disillusion the most optimistic balloon sculptor who tried to cheer you up. The kind of sad where the gulf between your house and the rest of the world is a dark, constantly shifting canyon (like the tectonic plates are wired up to your emotions). The kind of sad that makes puppies cower in fear of it (in fact, this happened to me a few times).
But it isn't insurmountable. It's just important to remember that you should never think you are anything less than remarkable.