As far as I feel, the most common mental health disorder is Anxiety and Depression. Anxiety is when one keeps worrying about the future and, in fact, starts living in the future. Due to the stress load and competition around them, most people are incredibly anxious, which takes a significant toll on their health after a while.
Another one is depression which is when one lives too much in the past. It's when one can't get over the situations of the past and lives in that guilt forever. Both of them should understand that this will only make their health worse. It is essential to living in the present and be conscious.