If we merely allow ourselves to react to whatever thought comes into our heads, then that’s the experience we have. If we sometimes feel overwhelmed and out of control, no surprise. When we take the time to be aware of and attend to what goes through our minds, we have different experiences.
The practice of mindfulness is a tool that assists us. We practice being mindful or aware of our thoughts without judgment or evaluation. Just notice without reaction. This practice strengthens us to become better people, which we call mindfulness.