"Usually I am not sad."
Translation:Zazwyczaj nie jestem smutna.
It's hard for any language to have 'no word order', even if it's relatively free. Your sentence means something like "Usually I am not-sad" with 'not' negating 'sad', and not negating 'being', as it should. Keep 'nie' before the word it negates.
Also in most contexts, 'nie' with an adjective in its basic form is written together: bezpieczny/niebezpieczny, duży/nieduży, etc.