What to do if one feels nervous or sad then their mind will resort in an obsession and compulsion?

Obsessive: Obsessive disorder has to do with thoughts that won't go away and preoccupy us, worry us. It is often linked to behaviors that we do to prevent some unknown harm from happening (compulsions). Together they are called obsessive compulsive disorder or ocd. It sounds like you were obsessed with some thoughts when. If that is still an issue it is very treatable by a clinical psychologist. Peace.