Hey.
I haven't been diagnosed with BPD, but I'm pretty sure I have it, and I can totally understand where you're coming from. A few years ago I had a best friend, and she started to get equally close with another friend. I got really attached and jealous, and I sent her a hate email about how much I disliked her and how I never wanted to talk to her again. I got my wish, and we were no longer friends. It took me awhile to even realize it was my fault; I blamed it on her for a long time. Sometimes I still do. But I think that for the most part, I've forgiven myself.
When you have a mental illness, it's beyond your control. You can't help how it affects you, and sometimes you can't even affect what it makes you do. It might not have been the most reasonable thing to do, but I can understand why you were jealous and I think I would have felt the same way. You may have overreacted, but it's not your fault. You wouldn't have chosen to if you didn't have a condition.
Don't beat yourself up about this. It's one thing if you do this to someone when you don't have something like BPD, but it's completely different when you have a reason like this.