First - joining this forum and reading / responding to other threads in addition to getting your own answers here is a great start. there are tools on this site as well which are truly exceptional.
A next step might be to get a couple of good books with a lot of practical information.
Stop Walking on Eggshells
and
Loving Someone with Borderline Personality Disorder: How to Keep Out-of-Control Emotions from Destroying Your Relationship
are two good books you could try.
you could also connect to a support group - here's a link to some you could join which are run by the author of Stop Walking on Egg Shells:
https://groups.io/g/MovingForward You can hear directly from other parents what they are struggling with and what is and is not working without having to wait for emails.
An absolutely must is to get access to someone who can help you with dialectical behavior therapy - that is one therapy that has clinically proven success with BPD that was developed by a researcher Marsha Linehan. You can read more and get access to tools to help at this site:
https://depts.washington.edu/uwbrtc/our-team/marsha-linehan/. If your child can get a therapist who has this in their tool kit that is superb, but you should also learn these skills to help improve your interactions.
Like you I just realized my adult son has this diagnosis after years of having a depression diagnosis. It is truly overwhelming to figure out what to do... but you can get a ton of support and start to get some traction. There is hope and BPD does have a reasonably good prognosis - it just takes much longer than any of us would like and there is not a ton of recognition of this disease out in the community yet so it does feel lonely in many ways.
You are making a great first step joining here and I am sending you lots of positive vibes in this scary and overwhelming time.
