The personal statement is also a great place to explain more serious problems, like less_than_ideal grades or test scores. Don't be afraid to write something like, "I don't test well" in the statement. As long as you can provide a good reason for you app's problems, it's okay to address them directly. You won't be able to hide it from admissions officers anyway, so providing a reason for problems is always preferable to ignoring them.
It is important to put yourself in the shoes of the Admissions Officer: why would your personal statement be unique, how are you different from the other candidates? Primarily you need to decide how you will divide your personal statement into digestible chucks. The most obvious way is to divide into paragraphs of 100 words. If you aim for between 500 and 600 words at most, it will help your admission officer when they have to read thousands of personal statements to find the perfect student.