I'm sorry things did not work out with Knight. I would have been irked by the Facebook post too. I don't get how a public announcement via Facebook would hurt you less than talking to you first.
The former ex not liking you is not your problem. It's hers. You can offer to speak with her to clear the air via Knight. Then it's up to her. But you certainly don't have to do this. Besides as time goes on, and you act like a friend around Knight and not a lover, she'll probably get over it.
You should give yourself a pat on the back, even though I doubt you much feel like it. Yes, mistakes were made. But you didn't shy away from looking at them closely, figuring out what happened, talking things over, and making changes. You'll make mistakes again but I bet they won't be the same mistakes. Lots of people much, much older than you never figure this out. Someone really smart told me that the point of life is not to avoid failure or never make mistakes, but to learn from the ones you do make, and to grow from the experience.