People with BPD can have very low self-esteem, and it can help if you remind them of good things about themselves.
A person with BPD is likely to feel emotionally unstable, so it helps if you can keep stable. You may need to find more support for yourself.
Your loved one may have more than average changes of mood and attitude, and feelings of being abandoned or let down by you. Try not to take this personally.
Try to keep in mind that the person you care about wants very much to be loved and cared for but may not know how to ask for help.
If the person you care about is not receiving any help or therapy then you could help them find information about therapies that might work for them.
You cannot give someone else a sense of self-worth. People with BPD have to find their own way to this, through therapy and hard work. But you can support and encourage them as they go through this process, which is likely to take time.
Learn as much as you can about the disorder and find out whether there are any groups to support families and friends, as well as the person with BPD.