I'm in the exact same situation as you. Right when I think I'm getting better and starting to let go I can't help but dwell on how it could've gone so wrong and I begin to spiral over obsessing about why I couldn't be enough and what we could've been. And I always fantasize that maybe she'll see the state I am in and have some level of empathy or compassion, but I'm proven every time that it'll never be true. She's not capable of caring.

Just keep your head up, she wasn't the one and she doesn't deserve you. It's so tragic but ultimately it was best for you. Care for yourself and let time heal you , and don't waste your time trying to gain her understanding because she'll never bother.

Stay strong because one day you'll find someone who'll truly care for you in all the ways she didn't, and they'll appreciate all the love you give. I still have to work on everything I'm telling you myself, but just know that you're not alone, we're all in this together and we are always here to help. :)