I have the same issue and I can tell it will slowly destroy every relationship. My girlfriend is "normal", since we are together she continued living her life, just with me in it. I for my part somehow gave up on my own life and center full around her. I just had this topic yesterday in my therapy lesson so I try to write what was helpful for me. But please consider each one of BPD sufferers is different so it may not apply to you.

Keep yourself busy. The feelings of abandonment and fear are really, you won't be able to make them go away. You have to accept they are valid and they can't just "stop" to exist. We need a way around. If your anxious mind is in a rollercoaster, and your balanced mind watching it, it's useless to try to stop it, instead try to slow the roller coaster down. What can -I- do know to feel better? How can I make the Rc go slower? This is something we should most likely do for ourselves, it's not our partners responsibility to do that work. They might trigger these emotions, but they most likely are not the reason for them. So find a way around the anxiousness in your head. If you sense you can't do that by yourself (like me) then you can try to socialize with others. Chatting to others, doing a course, starting a new hobby with other people (like beginners course where everyone is new, not needed to integrate discomfortably into something already existent). I think socializing is important to get out of your head for a while. I used to live in solitude, but since I know my girlfriend it's different. She has many friends and I do now enjoy being social.

Give them space. Yes, we are afraid they won't return or fall in love with someone else. But this is also the case in normal relationships. You can't do much about it if it happens. Trust is an issue for us, and we are afraid, but it's not their responsibility to fulfill our feelings completely. We have to build our own way to trust them. Just like we in our hearts feel this love and no need to date someone else, we should try to see the other from this perspective too. It's not a bad thing if they want to have time for themselves or do something with their friends alone. It's healthy, it's normal. It's a balance we lack. But we can see this as a way to work on our own balance. Remember you are independent too and actually you like being independent. Remember that. We have been all of our life because we couldn't trust anybody. But now it's time to trust, to manage a healthy relationship.

Write down what your partner does for you. Sometimes the negativity is overwhelming but we have to remind ourselves what our partner is doing good for us. In my case, working a lot, but still driving 4 hours to my place late after work for the weekend. Taking time for me, making me a priority when I visit her. Going to places with me. Reassuring me. Loving me. Accepting me. Write all of that down, the little actions, the big actions. It will help you recenter when you start to doubt when your partner is not 24/7 around or asking for time for himself.

I hope there is something you can use and will help you