Hi Shield-Me,
I hate it when some people in real life don't take it seriously.
I think when people don't take you seriously, it's because they "can't" for some reason.
Disclosure- I'm not sure if you will like what I have to say.

My thoughts are based on my personal experience, and they're just "my thoughts", nothing more.
I honestly suspect that when people react like this, it's because they may be uncomfortable with what is being said, and simply don't know how to help you or what to say. Borderlines can be really abusive. Telling those stories might be cathartic for us, but for the person receiving that info, it can be disturbing to hear. I think that telling other people "our truth" about a parent or sibling can make the other person uncomfortable, and sometimes they don't want to hear those messages. Also, unless they have a family member with BPD, it is not an experience they can relate to at all, so depending on the kind of person they are, they may try to empathize, they may be apathetic, or they may judge you. In the best case scenario if they actually try to empathize, they still can't really be "present" the way you or I need them to be because they simply have not "walked a mile in your moccasins". Most of the time they are more likely to judge you because their experience is seeing "the good side" the pwBPD displays in public. I think groups such as AA are so successful as support groups because the other people there share a similar experience and "get it", and also because their support methods are proven . So for these reasons, I refrain from talking about my uBPD to other people. In a weak moment, I once shared my story with a close friend who even has a family member with a different diagnosed mental disorder. Despite my friend and I being so close, I found it an invalidating experience because she had "prior positive opinions" of my mom (who she knows a little) and she just couldn't "understand" what I was saying because she has never experienced the kind of behavior we get from our pwBPD. She really tried to be helpful and do her best, but she just kept holding out hope that the relationship with me/my mom could get better. We all know a BPD doesn't just "get better". She's still my best friend. It's not her fault she can't understand what it's like for me. Honestly it's taken me almost a year to understand myself after reading a ton of books, online material, getting T, doing "work" on myself, so how could I expect her to understand just by listening to me? What I learned from that, was to get my support from my H (who gets it), my T (who is great), and this forum (people who "get it").
Also therapy helps, talking things out with someone who will not judge or hurt you helps. Sometimes friends help but they come and go. Pets will love you no matter what and a therapist went to school to figure out how best to help you. They care.
I think you've hit the nail on the head here. Figure out what works for you (eg T, pet,...), and stick with it.