Pessimism is smart; by expecting the worst you are able to plan for trouble and avoid it.
Which revision below is an active sentence?
Pessimism is smart; you are able to plan for trouble and avoid it by expecting the worst.
Smart pessimism expects the worst; you plan for trouble and avoid it.
Pessimism is smart; expecting the worst lets you plan for trouble and avoid it.
When you planned for trouble and avoided it, you were proving that pessimism is smart.