General Antonyms:
* Happy
* Joyful
* Cheerful
* Hopeful
* Content
* Optimistic
* Thriving
* Prosperous
* Enthusiastic
* Energetic
More Specific Antonyms:
* Hopeful (if forlorn means lacking hope)
* Cheerful (if forlorn means lacking cheer)
* Thriving (if forlorn means lacking prosperity)
* Content (if forlorn means lacking contentment)
The best antonym depends on the specific context in which "forlorn" is used.