Hello all, I'm new here. May I just say what a wonderful community this is. Well done. I'm looking for wisdom, advice & understanding; pretty sure I came to the right place. Hope this post isn't too long

I've recently made the most definitive step to distance myself from a partner I've been on and off with since early 2017. Rang her to say she makes me miserable and that for my own sanity I need complete space and finality. Then blocked her on everything. She lives far away from me right now, so no danger of face-to-face meets. I've done this before, though - more times than I can count. But... I feel more than ever that I want this time to stick. I also have far from 100% confidence it will stick.
Want to add at this point that the ex-partner is NOT diagnosed with BPD (but presents all the symptoms, and has made me feel like I'm living in an insane parallel universe since we met). She's currently getting therapy, on my advice/urging. So first question: anyone have any experience with
assuming their ex has BPD? I feel uncomfortable making this assumption, but by god it feels like she is BPD. I decided over a year ago she displayed BPD tendencies, but never told her. Thought it would be deeply presumptuous and rude - not to mention potentially detrimental to her already wobbly mental health.
For context: she's threatened suicide regularly; emotionally blackmailed; left me briefly for other men; sent letters, emails, begging; turns up at my door unannounced; pulled a knife on me; hidden my passport and ID; stole my car keys; rewritten our history and things I've said and done; regular verbal and physical abuse; the list goes on.
I've read a lot about NC here and elsewhere, and lots rings true for me. It's inevitable I'm days off a long email that twangs all the right heartstrings. This tired formula works on me incredibly well. I blocked her on email (and everywhere else). Her emails will go to my spam folder. I'm ashamed to say I'm checking that folder about 5 times a day. I blocked her number but my phone still tells me when & what she texts. I feel like I'm addicted to the drama, the next phase, the inevitable cycle of hope and pain - and I know that the full, wall-to-wall realisation that she's terrible for me and I MUST get away for good hasn't really landed yet. Like my being unable to throw away the trinkets and gifts she gave me, I feel really unable to *not read* a message she sends me. Like I say, I'm addicted to the drama (in the immortal words of the Black Eyed Peas...)
Then there's my own mental health, which has taken a sustained battering for two years. I'm sure the advice is to seek therapy. I know my own issues pre-exist this relationship and overlap with it. For instance, I've often dated mentally unstable and depressed women. Friends deem this a saviour complex. I'd go further and say my ego cherishes the opportunity to make someone happier. I think this borders on narcissism, even if I tell myself it's because I'm compassionate, nice, etc. I've largely forgotten what self-esteem feels like - perhaps that's common? I also suspect I've come to take emotional laceration as a display of love - and pain as a more significant part of life and love than perhaps it is. So, question also: do I seek a therapist with particular experience with BPD relationships, or go more general and have more general therapy? Has anyone kind of done both? I'm worried about the 'convenience' of unloading all my problems on this relationship - but also think 'general' therapists may not understand?
Finally, if you'll indulge me: how can I tell my friends and family what this relationship has been like? How have you guys communicated it - or is it even possible? All I know is my friends have watched with bemusement, bafflement and continual eye-rolling as the tumult took place largely behind closed doors - with 'we're not together'/'we're back together' flip-flops each time they ask for an update. I want to open those doors now, but not sure how.
Really grateful for any and all insights. I feel like the sun is rising on a new day and a new start, but I need your help to run towards the sunny horizon. I don't feel out of the darkness just yet.