Expect good possibilities while remaining prepared for difficult ones.

Planning for the difficult case does not require expecting it. You can genuinely anticipate a good outcome while having a workable answer for the bad one, and that combination is what allows you to take risks without being reckless. Preparation is not pessimism.

For something you are hopeful about, write a short answer to what you would do if it went badly. Then stop thinking about it and continue hopefully. Having the answer is what lets you set the worry down.