Half full or empty? Not so fast...
The truth is we need a little of both. It's the ability to move fluidly between pessimism and optimism that keeps us well.
Both pessimism and optimism can be strengths and weaknesses. For example, you need pessimism in the law or medical profession. We can't miss anything.
However, tip too much to the pessimistic side and it's miserable, perhaps partly explaining the high rates of depression and suicide among the law and medical profession.
Optimism is obviously more fun and feels happier. However, being overly optimistic can cost you. For example,
1) you trust that your kids would never be interested in drugs or sex so you never ask.
2) yes, the gas is on empty but you really can't possibly run out on the road.
3) you trust your mate when he/she says the suspicious texts were weird and have no idea how they got there.
This is simply an evolutionary balance between keeping us safe from danger (good dose of pessimism) and proactive to benefit from taking some risks (good dose of optimism)!
How to balance this fine line?
Practice Beginner's Mind: Look at situations, people, things as if you've never experienced it before. Remember, our brains look at everything relative to what we've learned. Peel back the layers, releasing the prior memories of that situation, person or thing, and all of your expectations. This will help train clarity in seeing situations for what they are and guide you to pick pessimism or optimism.
Example Exercise: Look, listen and experience a loved one, someone extremely familiar to you as if you've never met them before. See the color of their hair, eyes, the way the light bounces off their skin. Hear their voice and feel their emotions. Forget that they "tend to be this or that" forget "what they always say or do". Experience it anew. Notice and discover new things you've never seen before.