when you thought you already move on, out of sudden your past come back or even haunt you.. what will you do?

This danger is always present, so I am prepared already, when I leave the gaff. And I would ignore my Ex

It just means you still have to heal and deal with some things otherwise it wouldn't have come up again.
I notice that currently this seems to be prevalent and I think it has to do with Mercury being Retrograde in Pisces. I too have certain things come up, thoughts, feelings and so on.
It's the same if someone literally crosses your path again. If it affects you and upsets you, you still have some work to do on healing.

what if it's not about the ex but more to the past event itself? and sure, we thought we already prepared for the worst but still.. the shock, the "surprise" for the past that emerged make it more difficult to be healed or for us to really move on, somehow somewhat, I think.

Endure? Cope? Take responsibility? Ask for help? Communicate? Run away?

Kinda depends on what's going on.

I mean there's a huge difference between:
*knock *knock "I'm detective Murphy, you're under arrest for a hit and run from 10 years ago."

vs.

"Hi, remember me from that drunken night in college? Yeah...here's your son. He's 18 now and wants past child support now to pay for college."

vs.

My ex from 2 years ago, after a 10 year relationship, just messaged me on a dating site.

vs.

whoa. my current g/f just did something that reminds me of what an ex did and now I can't get my ex out of my head and I feel my current relationship is now going to follow the same downward path as I'm starting to notice and in hindsight realize a lot of similar personality traits.