Also often they have difficulty trusting others. They have a very high need for affection and loving security. They may also have difficulty tolerating periods they have to be alone and often have problems with object permanence when it is not (when a person disappears may struggle to remember the feelings I felt for her or the other person felt, so they usually want to keep that person objects periods of separation)

How should I deal with a borderline?

Try to be honest and clear.
Try not to damage your self-esteem as far as possible (but not lying), they are very sensitive.
Try to be fair and that both sides are equally involved in the talks, do not blame yourself all the same, not all blame the borderline. There is a middle ground where both sides should discuss and accept their share.
Although it is difficult trying to be caring and empathetic, give your support when you are wrong.
If the conversation goes hand let him know you want to fix things but you'd rather wait for him to calm down a bit, to postpone the conversation if it comes to insults. Do not respond to insults or attacks in these cases it is better to withdraw.
Above all, patience and try not to take things at face value, often say things they do not really think.
About your feelings and why you can not do what they want is clear. Showcase your reasons calmly.
Do not you face the situation alone, looking for help and support (both for you and for the other person).
hold him in their treatment and reinforces how well it does.