Hugs!
I hate to say it but you need to distance yourself from your friend with the BP/Borderline. I feel bad that she's obviously struggling but she seems to be the type who wants you to go down the rabbit hole with her. You need love & understanding.
What are some healthy rituals that you have helped you in the past? You may want to write a list of the positive things you have to look forward to; express your pain through poetry or art- just get it out of your system and let it on the page; maybe a letter to your 11 year old self letting her know that you fought some battles & continue to battle- but that you're stronger now.
unfortunately, I had to cut out of a friend who has borderline- may be bipolar as well- and she was trying to drag me down with her. I hit my breaking point. While I wish that she would get better, I've accepted that she doesn't want to try. Since then- my moods have really stabilized.
unfortunately there aren't any shortcuts. hope you take care of yourself.