A really fast thesaurus... with lots of synonyms.
Other words for sad include down, gloomy, melancholy, sorrowful, and heartbroken — pick one to match the depth of the feeling, from a passing low mood to deep grief.
For deep sadness use sorrowful, heartbroken, mournful, or despondent; for the most hopeless kind, desolate, forlorn, or inconsolable.
Down, blue, glum, and low all describe a mild, passing low mood without the weight of words like heartbroken or grieving.