The best antonym for pessimism is optimism.

It can be. People with depression do not have a positive outlook on life, but it doesn't have to be pessimism. It can range from neutral to hard core pessimism.

People who are pessimism are usually gloomy.

This sentence above is actually incorrect. The proper sentence would be:

People who are pessimistic are usually gloomy.

The proper way to use pessimism in a sentence would be:

Being depressed and viewing everything negatively is usually a sign of pessimism.

Example sentence - His pessimism wore on all of us.

Pessimism is a negative state of mind. It is seeing the dark side of things. A better way would be to use pessimism and optimism together to make realism.