Hey buddy my girl just told me last night she got an STD, yet I'm still not mentally ready to walk away! What more freaking signs do I need? How do I walk away man? Just block her phone number?
I know too little of your situation to really make a fair judgement. I mean, is she BPD? Do you suspect she has BPD? But more importantly, how long have you been together? Is this in the first weeks? If so, then perhaps the STD is from a previous lover and while you are in your right to walk away, you can also choose to wait for the curing of the STD and continue from there.
Are you together for a long time and are you sure she didn't get the STD from you? Then it is a big fat red flag that you can't just ignore. It means for 99 % sure she's been fooling around with others. You could even say you're lucky that she told you, as some BPD don't even tell it to their partners. But again, I know too little of your situation, but yeah, it would really be a big red flag for me.
Edit: You say you are not mentally ready to walk away. I take that as a sign that you've been together for quite some time and she hasn't been good to you, and the STD is just another problem in a long line of problems. You have to ask yourself why you are not ready to walk away. Is it because you are co-dependent on her? That's no shame to admit, many of us are / were. We have holes in our lives too and our partners were there to fill them up, only problem is BPD filling your holes up is extremely dangerous as they will always leave eventually, leaving you with the pain of feeling those holes all over again and being extra sensitive to them. You have to start to work on yourself, identify why you don't feel complete without your partner, and try to start work on that. It will make you stronger so that you can live a life without them if you choose to.
In MY experience, you can work on that after you've seperated with them, not during the relationship. To each their own, but I couldn't do it while being with them.
If you DO want to walk away, you have to analyse the situation and judge what is best. In my experience it is best to just dissapear, which is easier if you are not living together and without child together. Just cut the contact and block her on everything. Or send one last message that you are choosing for you and that the relationship is over, without giving her a chance to suck you back in with threaths or begging.