I agree that positive thinking and trying to turn negative thoughts around with such can definitely help with your mental state.
Though I don't see "no" as a bad word either. Especially when it's said to prevent people walking all over you. I'd often find myself agreeing to stuff that I'm not totally investet to just because I'm afraid that I might hurt people by declining. Sometimes I'd say "no" initially but will be swayed with enough nagging. But in the end I'd just feel not taken seriously, like my opinion and wishes just don't matter. So, a loud and clear "No!" is actually really necessary sometimes. I might feel a little guilty for brushing them off like that but saying "no" to something and being taken seriously, for me, gives me a great feeling of empowerment and self-confidence.

The worst word, or rather phrase, for me is "not good enough". It just makes me feel worthless, even and maybe especially when I've put a lot of effort in something.