I'm not expert but I feel like you haven't set strong enough boundaries for yourself.
My partner knows I would just be gone if she cheated. I made that explicitly clear when she asked me my attitudes on cheating. She's Japanese and I live in Japan, it's perfectly normal here for husbands to go cheat as long as they never admit it and keep it secret. Also it must never be brought into the home and disrupt the 'harmony' of the house. Wives treating their husbands like ___ (even without BPD) is perfectly fine though, oddly!
Anyway, cheating issues aside, which realistically can of course be forgiven; dangerous behaviour must be a line in the sand I feel. I think if I was exposed to dangerous driving due to mental illness I would have probably demanded the car was stopped and I would have got out and left. If you don't highlight the boundaries properly surely it will start to seep into the general 'acceptable' behaviour of the relationship?
We can still have empathy, love and a desire to help with everything we do... .but I feel we all tolerate too much sometimes. Protect yourself emotionally and physically from her illness. You already have depression and you're being painted as the enemy within. Boundaries. Put a plan in place to deal with severe boundary breaking.
I knew a boundary was going to be broken about a month before it happened. Doing everything I could to validate, love and support but it just kept escalating and occurring more and more frequently. I slowly and casually shifted stuff out of her apartment to my own, called her friend and informed her of everything. I let her friend know I expected something dramatic to happen soon and I may need her to look out for my partner (from a suicide point of view). After the past few time I realised I couldn't be the rescuer for the suicide threats. It's a powerful and disturbing tool to get your sympathy and attention. I had to remove myself from that cycle.
So anyway, a few weeks later, after the 6th meltdown in a single weekend she snapped, trashed her apartment and went for some scissors, threatened herself or me (it wasn't clear). Boundary crossed. I walked and she understood why after many tears and battling to stop me leaving. Her friend is making sure she gets help. I can't be responsible for her therapy, as you say... .it's shaming for them. Anyway... .things are improving now, she's getting help and is showing her best face to me when we see each other. She needs time, routine and realisation there are boundaries that cannot be crossed for a relationship to function for the good of both people. If she can't do that, so be it. We can only control the boundaries and our own actions. It's up to her to acknowledge the irrationality of her behaviour and make an effort to correct. We can only draw attention to that unreasonable behaviour by having strong boundaries. Weak boundaries normalise the behaviour.
Put a plan in place to walk away (not end the relationship), you need a safe place to enforce your boundaries. Just a suggestion. Only you can know if you think that'll work for you. I'm sorry you're going through this. Sometimes it's just about experimenting with strategies... .god knows how many pieces of 'good-looking' advice I've followed that haven't had the desired affect.
All the best!
