There's not a lot of space for a lot of stories. That doesn't necessarily mean that you have to limit yourself to only one, but it does mean that I don't want to be inundated with every life event you think of as significant. Tell me something that will grab my attention and make me interested in knowing more. Make sure your transitions flow easily so I can follow your train of thought. (I suggest reading the PS out loud to yourself until it sounds like something you would say). I would keep it positive and not dwell on the perceived shortcomings in your medical education or clinical training. You can state what you are looking for in a residency program and let it be ambiguous as to whether it is because you consider those elements to cater to your strengths or to strengthen weaknesses.
If, as you read the PS to yourself, you think something along the lines of "Hey--this is an interesting person I'd like to meet!" you're golden. If, on the other hand, you think "This person sounds arrogant/cocky/self-absorbed, etc," then you need to go back and modify your writing style.