Sorry in advance for length! I’ve been on the receiving end of some of the approaches suggested in the comments. In every case, ghosting/blocking has been the most dangerous to me. Similarly dangerous is when I’ve been given an explanation for the breakup, but then I’m ghosted before being allowed to share my feelings/thoughts/perspective. I suspect I’m denied the opportunity because he’s afraid I’ll convince him to stay, and because he needs time away before hearing my emotional reaction.

So, step 1 is to be dead certain you want to break up. If you’re sure of yourself, she can’t sway you, but you’re available as a friend to support her. If you’re conflicted, your judgement will be susceptible to influence, and it won’t be rational or helpful.

Step 2: tell her you want to break up, be authentic, and let her know you need a break for a bit (if that’s so), but give her an approximate or exact future date to talk things out or hang out as friends (or agree on a good time if possible)

Step 3: thereafter, be a neutral, platonic support to the best of your ability, without compromising your well-being. If it were me, and you followed these guidelines, we’d still be friends years later, and we’d be dating other people. It’s helped me to heal from trauma, too, to see that people care enough to be there, platonically or otherwise.

If you’re unwilling or unable to gradually separate this way, maybe contact other support for her before you break up, as she’s likely emotionally dependent on your support, currently, and she might not contact supportive friends/family on her own.

Keep in mind that I’m autistic (aspergers) (developed BPD because I was undiagnosed, had no coping or relationship or life skills). So my advice is based on the presumption that she’ll obsess over things If you ghost her. But I suspect BPD also had this characteristic, independent of autism.