Fear (from abandonment) is almost everything that goes on in the brain affected by BPD. It clouds everything else. That's why they seek replacements, induce memory "blackouts," and do all the other things that you read on this board.
It is a pitiful state of mind, and something that nons find difficult to relate to. I can't imagine myself eve thinking that way. Grieving is a very normal and human thing to do. If someone cannot grieve (in whatever way there is), then there was no love involved on their end. That is a difficult thing to process and accept, and something that I am currently struggling to accept. But it is what it is. My story is actually very similar to yours. I had enough dignity and self-esteem to call it quits despite my ex trying to continue. Actions must match or go beyond what is said. That was not there in my case. I realized that I didn't owe anything to someone who lied to me, so I started NC after that - no explanations, no goodbyes. If someone wants the gift of myself, I want the same from them. I don't want fears, lies, excuses. BPD or not, I want the truth, on time, and we go from there. There is nothing wrong to stand up for yourself and detach from toxicity. It's a road with thorns, but think of walking a mile on thorns versus years or, worse yet, your entire life. The choice is clear, and there should be no regrets.
Venting out is sometimes healthy, as you can see

. Give yourself a big hug and smile, even if it's hard! It will be easier as time goes. It's still very fresh on me, but I know I made the right choice. Right choice is usually hard, but a whole lot bettet in the long term.