Oh my goodness I was just about to google 'necklace behaviour' thinking it might be some weird deviant sexual practice. 
I am sorry, this is a shock and of course you are finding it hard to deal with. It's understandable, as it's not a very attractive label to have slapped on you, and one that many will not have much empathy for, or understanding of.
I am not an professional/expert but I do know a bit about BPD, (for reasons I wish I didn't have) and I think most people with BPD have had trauma in their early childhood that affects the way they relate to others. Or at least most people who have had trauma in their childhoods will end up with something akin to a BPD, anyway.
Which is very sad of course, but the awkward truth is that many people (not all) with bpd do go on to seriously lead total car-crash lives where they damage others in a cycle of dysfunction, so it can be hard for someone who has been on the receiving end of that (as I have, indirectly) to see it as an illness or a vulnerability on the sufferer's part rather than a serious character flaw, or a get out of jail free card for awful behaviour. Of course like all forms of psychological disorder/mental illness it is very complex to unravel and is not as simple as x= vulnerable victim, y=nasty toxic person.
I think you need to spend some time getting to know your diagnosis in all its guises, and learning to recognise where you fit into it, and it into you. Not everyone with BPD will be the same. BPD will mean that you struggle to regulate your emotions and to temper your responses to stressful situations, and that you probably have some boundary issues. (I'm sure you are already well aware of that!) That is not to say that you necessarily damage others with your behaviour; perhaps the only person you harm is yourself. But you need to be honest with yourself. There is probably a little bit of BDP in most of us - it's just a bit more obvious in some than others!
Try not to see this diagnosis as a harsh criticism of you, but instead look on it as an explanation for why you do certain things, and use it as the key to understanding whatever emotional baggage you carry around with you that weighs you down and affects your quality of life, and work forward from there.
But I would just like to add that whatever caused this in you, no matter how difficult for you it has been to deal with your own issues, if you have caused major problems for others with your behaviour then PLEASE PLEASE own it, accept it, and work on it, for their sakes, as well as for your own. I think that recognizing that will be an important part of helping you to get better. Being in denial is not good for anyone. Good luck.