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Memorizing and reusing a single password is the worst thing you can do. The number of leaks and brute-force attacks are increasing, and you should really be generating random and unique passwords per website if you aren't.
To get an idea of how easy it is to crack a leaked password, download a public list of a website you know[1] (ex. L1nk3d1n), you'll probably find the password you used at the time of their leak in the list, since ~97% of them have already been cracked.