Melancholy?

I find that the happier I am, the more prone I am to feel down almost immediately.
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It's like, if an emotion is strong, all other emotions will flare up as well, and I'll feel happy, sad, jaded, satiated, many emotions at once.
In that sense, I prefer being nonchalant...
However, I do feel that I lack passion in my life.
I feel that I'm not competitive enough, that I don't strive to be what I'm capable of being.
No, I know it; I know that I a much better person than I allow myself to be, and the only person to be blame is obviously myself.
I've known this fact for years, but still, no effort is made.
Another issue is that I don't get inspired easily, and even if I do, it's for a very short time.
The reason behind this, I dare say, is that when I was young, I didn't want to become an easily influenced or swayed person.
In a way, I think I fear passion.