Above all, mindfulness practice involves accepting whatever arises in your awareness at each moment. It involves being kind and forgiving toward yourself.
Gently redirect. If your mind wanders into planning, daydreaming, or criticism, notice where it has gone and gently redirect it to sensations in the present.
Try and try again. If you miss your intended meditation session, simply start again.
By practicing accepting your experience during meditation, it becomes easier to accept whatever comes your way during the rest of your day.