I think...
think... that while medications can be helpful in raising/stabilising mood, the best treatment for BPD is through therapy... challenging the thought processes... finding the 'true self'... well, there are a bunch of theories. Sometimes it's difficult to pinpoint what exactly is working and why... and what works for one may not be the way for another...
I've felt the way you do, and whilst I'm feeling somewhat better and more stable lately, funnily enough I did tell my T just last session something like 'No wonder people get off their faces on drugs' - I was feeling like everything was futile, and I had the whole Trainspotting theme running through my head, you know? 'Choose life. Choose a job. Choose a career. Choose a family, Choose a ****ing big television. Choose washing machines, cars, compact disc players, and electrical tin openers...'

Sometimes it's hard for me to try and get out of that mindset.
Getting back to the meds, though, I tried a BUNCH of different ones with no success, and in the end my doc got me on one of the older tricyclics, and it made a big difference for me. Apparently they can be more successful for treatment-resistant depression... so, maybe that could be worth discussing?
Like LolaCabanna said above: trial and error, unfortunately! I think that can go for boths meds
and therapy.