When writing your cover letter, keep in mind that the reviewer is only interested in one thing: the facts. Do not think of your cover letter as an autobiography. it should be brief and to the point. The purpose of the cover letter (and resume) should be one thing – it should demonstrate that you meet or exceed the requirements listed in the job description. It should demonstrate that you re interested in the position, and that you re available to accept the position if offered. Additional information beyond this can be counterproductive, as it dilutes the core purpose of the cover letter.
When writing the cover letter, avoid negatives. A cover letter is not the place to explain why you left or are leaving an employer, why there are gaps in your employment dates, etc. These "negatives" are best delivered in person during the interview so that your personality can counter them.