Don't be too hard on yourself, people get emotional.

I guess cutting must be an upsetting issue for you. For me (and I have BPD too), its not really a big deal at all. I used to cut, and most the people I know have cut, so I guess its something I am desensitized too!

Just communicate your friend that you are sorry you got upset, and that you love and care about them, even if you have to write them a note!

Your friend might have BPD, people with BPD often end up friends with other people with BPD!