There are no such things as bad omens..

Some of these omens are more than mere association. The black cat pictured at the head of the article would have been seen as a potential evil spirit, a wizard's familiar in corporeal form. It therefore represents an encounter, perhaps intentional, with a powerful antagonistic magical force. Another such example is the breaking of a mirror. In the past, a mirror would not only have been an extremely valuable object, but could also have been a magical appliance. Its destruction might unleash the energies bound within or prompt a curse.

Obviously, most if not all of the time, a mirror is a piece of glass and encountering a black cat only dangerous to mice and birds. Historically, though, there was a cause and effect relationship perceived, based on valid cultural memes. The connection is not irrational to someone who believes that a black cat is a magical manifestation. Now, if you don't believe in wizardry, it is a pretty silly practice to repeat.

I agree with the general drift of your argument. There are a lot of things you can do to yourself psychologically. I do believe in the existence of omens, however. I just think you have to be informed and critical when you decide to accept supernatural phenomena.