These things happen and often for no reason.
I suffer from BPD and also a few other related issue (including a severe anxiety disorder) so I feel for you sweetheart.
As always people will tell you ways that work for them, but they won't always work for you, and this is an age old fact of life. I'll share my coping strategies in the hope that perhaps they will help you.
I enjoy writing, always have from a very young age. Writing is a way to release thoughts and feelings without repercussions. I'm not saying a diary or anything, but sometimes just sitting down and writing/typing out everything that is on your mind helps. I used to find that if I typed out all of my problems and fears, anxieties etc and then deleted the whole lot it made me feel better, as if I was deleting them from my life.
Get busy! Nothing is worse with anxiety then not doing anything, it causes your brain to wander and that wandering makes things worse! Get a hobby or find something to be doing. I tried sport, scrapbooking, housework, writing, painting etc all helped because my mind didn't have time to wander from the task at hand.
Lastly, find someone to talk to, whether it is here on BnB or a councilor or a stranger in the street, find someone who is willing to listen and not judge. It really does helps when someone just takes the time to hear your problems.
I hope these help, as I said somethings work for some people. My inbox is always open if you wish to have a chat, goodluck sweety