I wonder if this explains the reason why I'm usually happier when I don't focus/constantly ask myself if I'm happy. The theory would explain it like so: I ask myself if I'm happy, and I notice all the things that are preventing me from being optimally happy. As I keep asking myself, I become better at noticing those things, thus becoming less happy.

My intuition is that you ask whether you are happy because you doubt that you are. So you already start negatively primed.

Instead, try to remember three good things that happened to you recently, and ignore everything else. Even better, write those things on the paper. Every day. (This idea is from the book "Learned Optimism" by Martin Seligman, and this article seems to support it. The theory is: if you focus on something, you remember it better; what you remember guides your next behavior.)