Hi there olyly94,
My therapist has told me that the loss of my former friend, who seemed to have BPD, is something that I need to grieve. I've realized, however, that I don't really know how to grieve in this situation. but none seem to help in this kind of situation, when the person is still alive but has caused a lot of pain and can no longer be part of my life.
I join
LuckyJim and
BeagleGirl in welcoming you.

I'm really sorry that you're going through this and you've seem to be at a loss as to what you should be doing.
I'm glad you're seeking help through a T and beginning to understand that grieving is healthy.
I've attached a little read which was published on Psychology Today, which may give some possible suggestions... .But really, only you would know best how to grief.
https://www.psychologytoday.com/us/blog/here-there-and-everywhere/201602/navigating-grief-how-copeAnd to quote "
Tell your story to those who will listen. If you have to find a professional to listen then do it! Seeing a counselor in times of grief is a strength. Try not to see it as a weakness and you will reap the benefits! Give yourself a break - grief is hard! It takes a lot of mental energy and sometimes other things will get dropped or forgotten. Mistakes will be made. Try not to judge. Instead be really kind to yourself and to others who are grieving. Give yourself time - LOTS of it! Grief does not end after 6 months, or a year, or even longer. For many, grief takes along time and it is hard. Do not listen to anyone who tries to tell you how or how long to grieve. Great love often requires great grief. You can grieve deeply and also learn how to thrive again at the same time. Take time to honor the pain and also take time to seek beauty and joy. Over time the pain will lessen. If it does not then have the wisdom to seek support."
We all deal with grief in our own unique ways, i played music and sang my aching heart out to those songs because i suppressed my own grief.
Do what you know best to feel your feelings, we're here to listen. Takeheart and takecare.
Warmly,
Spero