The mistake I made, was in thinking that the opposite of being happy was being depressed. In other words, I was thinking in black and whites and not allowing myself any gray areas. I realize now that it isn't a question of pure opposites. When you aren't quite happy, there are a number of moods that you can find yourself in and none of them have to be depressed necessarily.
You can be angry or irritated or contemplative or melancholy, but in all these moods you can address important aspects of your life that get neglected when you are only in a "happy" mood. Or when you force yourself to feel elated or joyful. For example, when you are in an irritated mood, you may finally do something about some issues that need your attention that you have been neglecting, because you have been too mellow and "happy" to pay attention to them. If you're angry, you may rectify a situation that is unjust and that has been nagging at the back of your mind for some time, but that you've been feeling too good for to rectify.
Even if you only solve the problem in your own head, it will bring you a step forward to gaining peace and understanding. You don't have to go out and settle scores with everybody. You don't immediately have to have the answers to all the problems. The point is that an issue has made itself known to you and you spent time contemplating it. Sometimes that is enough. If there are practical solutions, then apply them, but don't be like an avenging crusader with a sword. Don't set the world to rights.
The point is, though, that being not happy doesn't mean automatically that you're sad. It just means that you're something different and there's nothing wrong with that.
Well, that's all the wisdom you're going to get out of me this night. Of course, I've slept already and am up again, but I'm going to bed again shortly. At least, that's the plan. Until that time I may find all sorts of interesting things to do.
Have a good morning when you wake up, or a good night is you're still awake.