I understand you pain, I honestly do.
But this is coming from a supporter who is, it's not unreasonable to assume, may end up being on the receiving end of a relationship termination in the near future.
There's not a chance in hell that if she breaks up with me and dumps me that I will give even a slightest ounce of attention to make sure she isn't in pain. My obligations to her terminate the moment we are no longer together, the fact she is in a PTSD spiral is even irrelevant at that stage, when you are rejected and discarded the very last things on your mind is "how can I make this easier for the other person?" I may love my (God knows at this point) girlfriend, but if she wants me out of her life, well I'm sorry, I don't care if she has a descent into misery and is in pain or hurt, if she wanted me to then she'd not have cut me out of her life.
They just went and made it very much easier for themselves by ridding themselves of you, harsh but true.
You're expecting some sort of explanation and proof of a love that you'll never get, if he was unfaithful he obviously didn't regard you that highly in the first place anyway. He isn't going to fight for you because for one you vocalised you want it to be over and two to be honest, he obviously doesn't care all that much.
That isn't a reflection on you, but you are expecting something from somebody who isn't in a position emotionally to give you it, you're frustrating yourself and fighting a losing battle that is only going to cause you more anguish and pain. My advice? Focus your energy on WHY you chose to terminate the relationship, it was toxic, he didn't care enough about you, his PTSD is no excuse for cheating. You're not likely to get closure, so make your own, it's the best for you.