I have a certain amount of experience of this - not with a partner, but with a friend. I had some excellent advice both from another, older friend with BPD who had managed to grow out of the worst of it (this does happen btw) and also from some of the great websites about BPD.
Firstly don't walk on eggshells.
In my experience what helped my friend was not under any circumstances to alter my behaviour or react to her behaviour when she was having an episode.
If she started threatening suicide - and she did, regularly - I would calmly but firmly ask her to leave my house, even if it was the middle of the night. I would insist on it. It broke my heart but I would make her leave immediately.
If she self-harmed -and she did frequently - I would call an ambulance, and leave her to them. I did not engage with her about it at all.
Most of the time she was a marvelous person, really wonderful. A very good friend.
If you are allowing yourself to be manipulated by the person with BPD then you are doing them no service at all - you are not being a 'safe' person for them to be with. They need to be with someone who will be utterly impervious to their ups and downs. This is what they actually need, that sort of stability. They need someone who will not be manipulated. If you can be manipulated that will make them very anxious.
If you can manage to simply and totally be impervious to being sucked into whatever is their ongoing train wreck, then they will still have episodes and your life will be still full of drama. However you will be actually helping them, and also - and this is even more important - protecting yourself. If you can't do this then you need to get out immediately, for both your sakes.
It isn't your place or job to not trigger them. Someone with BPD will play you like a fiddle at the same time as being propelled into worse behaviour because you are making them anxious by your reacting and being 'nice' or whatever you think you are being.