Actually mindfulness is about NOT blocking thoughts. It is about acknowledging a thought as you have it (right now in the present) and letting it go without ruminating or judging the thought. As you continue to refocus on your breathing and don't hold on to your thoughts you do get much better at recognizing what you feel and think and of not letting it overwhelm you or trigger a series of thoughts. I assume that in the case here the idea would be to accept the uncomfortable thought of not wanting to breath as just a thought and not judge it as being wrong or right and then let it go and refocus on your breathing. I don't really understand how repressed stuff comes into it...but it does seem that learning to accept your thoughts and emotions without judgment would be a great skill to learn before trying to bring up repressed emotions!