If she is your best friend, that implies that you have some trust in her. (I say that knowing that BPDs have trust issues.) I would hope that you could tell her about your Dx, but you should be able to sincerely apologize for walking out on her even if you don't end up confiding this. You are correct in that she is probably making wrong assumptions based on your reaction, that it's about her, and not you. Maybe you could just show her one of your scars, without saying anything. Idk. I don't have any friends right now and I'd hate to see this create a rift in your friendship when you both need each other. And you'll hate yourself until you make this right, or at least make your best effort.