@Worriedmum1975 Best advice I can give is look after yourself first, and keep your boundaries. Love her, but keep your own feet on the ground. I'm afraid that a certain degree of cool headed distance may be necessary.
Most of all if you can and if she will accept it, get her some Dialectic Behaviour Therapy with an experienced therapist.
Do you know why she developed it? (asking if you know, not asking you to say here). In the past the main causes were thought to be abuse, chronic instability or severe neglect, plus a genetic predisposition - "genetics loads the gun, environment pulls the trigger". It certainly isn't always an issue in the parental home - unfortunately, it can sometimes happen that an adverse event that even the child themselves does not remember has effects later on.
The good news is (at least when this was something I had to deal with) that the prognosis is generally good as the person gets older, and very much so if they engage with treatment and -want- to get better. It's an extremely painful illness, so often people do. Having said that, my biological mother did not get better but with the background she'd had and then the disastrous marriage she had near to no chance. But I was told quite categorically that generally BPD/EUPD can often get better.
In short;
She can probably get better
Try to get her treatment.
Make sure, really make sure, that you do not burn out yourself.
Sometimes, and I say this with great compassion, no matter how much love you give her, it may not be enough. But it will be one of the positives in her life that can ground her, even if it doesn't feel like it when the storms are ripping all the good things in her life to shreds.
There will support groups out there for people who have someone with EUPD in their near family. Keep your head on straight - certainly when it was something I had to deal with, not all the professionals / support people were good - but some peer support can really help.
OP it sounds like you have insight and a strong wish to get better and really love and appreciate your partner. Those are really good signs. If you can get DBT and really work at it. Very best wishes - I wish my biological mother had had your wisdom.