Some people think mindfulness is about emptying the mind in some way. Wrong! It is not about freedom from thinking, but freedom with thinking. Thoughts will always come and go; mindfulness helps you relate to thoughts differently and work with them more skilfully.
The purpose of mindfulness is not to relax; it is to cultivate the skill of noticing what happens in the mind and body however it is: when there is discomfort, anger or stress as much as happiness, joy or ease. By noticing these things as they happen, you create the freedom to choose your response.