Yes, mental illness involves hard-to-control thoughts, but those thoughts are generated by the brain.
The brain is part of the body, and it can have issues just like any other organ.
In fact, the brain is recognized as the least understood organ in the human body.
Mind problems often aren’t as obvious as a broken arm or a sprained ankle because they are internal, but they are very real.
When someone is mentally ill, telling them to "stop worrying" or to "cheer up" and expecting it to help is similar to telling someone to "stop having a broken leg" but not giving them a cast.
People sometimes need to go to doctor's of the mind (psychologists and psychiatrists), just like they sometimes need to go to the regular doctor.