This is my theory, for the borderline person.  Think about these statements:

* Someone made me feel great, they must be an incredible person!  I idolize them.
* This person I love did something mean, and it made me feel awful.  I must make them suffer.

The problem with these statements is "made me feel".  Your feelings come from you only.  When someone else does a good or bad thing to you, in reality it's generally pretty minor.  It's your own feelings that blow it up into a Big Deal.  And when your feelings are improperly attributed to someone else, you give them all the credit or the blame.

People are responsible for their actions, NOT for how you feel about their actions.  When you think about their actions, subtract how you feel about the actions, and only consider the actions themselves.  You will see that this person isn't wonderful or horrible, but shades of gray.

There, I solved BPD.  Now somebody fix my depression.