I choose to think positive thoughts.

Thoughts arrive unbidden and always have. Nobody has ever chosen the first one, and treating its arrival as a personal failing is a needless extra weight to carry.

What is genuinely open to choice is the second visit. Which one gets rehearsed on the walk home. Which one is repeated to somebody until it hardens. Which one you go looking for evidence about.

That is a smaller lever than it sounds, and it is still a lever. Nothing knocks at your door by invitation. Plenty of it stays because it was made comfortable.