I'm also borderline and have had a similar relationship with my ex and my son's biological father. I thought life after he left me and my son was totally over. I also cut and obsessed about it and did other very unhealthy things. I had a major transference issue. That was almost 4 years ago, but honestly after a few months of therapy and getting healthy coping skills in dealing with that relationship, I felt a lot better and eventually got over our relationship (he doesn't see my son but that's a whole different issue altogether). But I remember the Constant texting and just wishing he'd hug me or anything at all...but to people like us with BPD, that does way more hurt than help.

Switch therapists and tell them everything about your relationship with your current one and get some healthy coping skills but most of all put as much space between you and your T as possible. It's gonna hurt and it's gonna feel terrible but in the end, you will be much better off and find someone who makes you feel happy and safe and that you love and you'll be able to handle that relationship much healthier