"Mixed Personality Disorder" is a colloquial expression for Personality Disorder NOS (Not Otherwise Specified). What it means is that you (or the person who was diagnosed) are suffering from an array of symptoms that fit a variety of personality disorder criteria, but don't meet the complete criteria for a single pathology. That sounds kind of scary, but it's really a "1 from column A, 2 from column B" kind of thing. So, you may be experiencing the paranoia associated with Paranoid Personality Disorder, but not the delusions and morbid distrust, along with the impulsive emotionality of a Histrionic Personality Disorder and the object rage of the Borderline Personality or the reactive rage of Complex Post Traumatic Stress...but you don't meet enough of the other criteria for any of these to be a complete diagnosis.
NOTE: Both the American Diagnostic and Statistical Manual (DSM) and its international counterpart, the ICD, maintain a "Personality Disorders Not Otherwise Specified (NOS)" category.
But they strongly disagree as to what should be covered by this catchall phrase. The ICD, for instance, relegates the DSM's Narcissistic Personality Disorder to this heap, insisting that it is not a full-fledged personality disorder.