How do you cope when your FP has a different FP?

I made a new friend and we've been friends for a little over a year. We are both diagnosed with BPD and it just so happened we became each other's FP, platonically.

However, my FP no longer sees me as their FP and someone else now owns that title. (It's been almost a month now.) We would hang out everyday and even be on almost 24 hour calls. They've stopped talking to me as we used to and they're always talking about their new FP. I want to be happy for them because I'm glad that they found comfort in someone. But I just... can't stop the overwhelming feeling of dread, pain, everything. It feels like chest is going to explode by how much sadness and anger I feel. I wish I could just be happy for them without feeling like im grieving.