General Antonyms:
* Optimistic
* Hopeful
* Positive
* Cheerful
* Sanguine
* Confident
* Upbeat
* Enthusiastic
* Idealistic
* Faithful
More Specific Antonyms:
* Realistic (if pessimism is based on an overly negative view)
* Pragmatic (if pessimism is based on a focus on negative outcomes)
* Trusting (if pessimism is based on a lack of faith)
The best antonym for "pessimistic" will depend on the specific context in which the word is used.