General opposites:
* Cheerful
* Happy
* Joyful
* Hopeful
* Optimistic
* Enthusiastic
* Content
* Satisfied
More specific opposites depending on context:
* Hopeful (if forlorn implies a lack of hope)
* Confident (if forlorn implies a lack of confidence)
* Loved (if forlorn implies a feeling of loneliness or abandonment)
* Prosperous (if forlorn implies poverty or hardship)
The best opposite will depend on the specific context in which you're using the word "forlorn".