I guess this person was one of my FPs, but it changes so frequently that I’m not totally sure.

We were best friends for 8 years, and they’ve been there for me despite my various mental health issues. Recently mine started to spike last year, and things became tense with basically all my friendships. For this one, I felt that despite my mental state he started to take advantage of me (only talking to me when he wanted advice or to rant, but if I wanted to do the same thing I would get ghosted). It happened a few times and I would calmly express that it hurts my feelings and he would apologize but it did not really get better and was an ongoing issue. 

One day I was having a particularly bad day, I got in a fight with my abusive stepdad and texted him for support. Basically I didn’t get a response and then his late response was something that was really insensitive/rude about the situation. 
I just couldn’t hold it in anymore, I said many horrible things but it was mainly how I thought he was a piece of shit, I thought they were garbage,  pretty much any mean thing you can think of I probably said it. And that I never wanted them in my life ever again because they’re toxic. I would apologize a few weeks later, but then I quickly took it back by saying “what you did was still wrong though” so basically my own apology was still impulsive and self absorbed because I was obviously still mad about the situation. I called them toxic yet I acted toxic myself. I felt the need to tell them how much I wouldn’t need them in my life anymore multiple times. 
I don’t believe anyone should stay friends with someone who treats them badly, but I wish I would’ve done a slow fade or ghosted or done any other situation. I chose to hurt the other person on purpose instead so they could feel how I felt. 

I think I’ve ruined any chances of a real apology now or smoothing things over. I tried to message them a few weeks ago (genuinely telling them I hope their life was doing well and I regret how I reacted) but I didn’t get a response. 

This situation happened last spring around May. I still feel guilt. I still feel like a horrible person. I wish I could take it back. I have so much guilt for exploding when i was doing so good with controlling my emotions and my temper for about a month or so. I know that mistakes don’t make up a person as a whole, but I still feel defined for when I lose control over my BPD, because it brings out the worst in me. I wish I could only hurt myself and not others. 

(Also before anyone asks, I’ve been in therapy for almost two years so it’s not like I’m not getting help)