Is openness to experience the same as flexible thinking? Probably not, but closedness to experience is certainly farther away than openness. Although the research reported below targets older adults, I would guess that the same process works for younger adults too.
"Abstract: The present study investigated whether an intervention aimed to increase cognitive ability in older adults also changes the personality trait of openness to experience. Older adults completed a 16-week program in inductive reasoning training supplemented by weekly crossword and Sudoku puzzles. Changes in openness to experience were modeled across four assessments over 30 weeks using latent growth curve models. Results indicate that participants in the intervention condition increased in the trait of openness compared with a waitlist control group. The study is one of the first to demonstrate that personality traits can change through nonpsychopharmocological interventions."
For example: If you're locked into negative self-talk, learning inductive reasoning (aka problem-solving thinking), doing cross word puzzles, learning Spanish or kick boxing, might bring about the similar flexibility of thinking and openness to experience. And, encourage you to be more hang loose, flexible enough in your thinking to say to yourself, "Let go of this silly waste of my time; negative self-talk. I'm smart, a realist, and though not perfect, I'm a sharp cookie. Give NST a break, give it up and move on." Change creates change. New experiences create new perceptions. Acquisition of new habits helps eliminate old habits.