I never knew about BPD till toward the end, which is the reason I dumped him, it became obvious that a r/ s with him would be an exercise in futility.
This is probably at the core of it. You went through madness and left. still wondering what was all that about and frustration with lots of unanswered questions. I was close to this too. Once discovering the problem, I took a mindset that I was staying as opposed to undecided.
What this meant is that I tried everything possible, climbed the impossible mountains, tried every trick, and really got to know the world of BPD. I am still in the staying mode, but the point I am making is that if it had, or does fail, as many still do, there will no unanswered questions, no" what ifs". I have undergone an enormous amount of self examination and I am a far better person now than I ever was.
None of this is a guarantee of anything, but to answer the thrust of your post having gone through this process if it fell apart it would be a lot easier to let go and let the resentment subside.
You will find most of those on the leaving/Coping and Healing Boards who are the most wounded comprise mainly of members who are discovering about BPD after the event, rather than having first exhausted all attempts at repairing with the correct tools.
Non of this helps you, but it may explain the difficulty you have in letting go.