How does that happen?

Can anyone help me understand how someone can go from:

I'm so sorry I lied and hurt you, I'll do anything to fix this, you're my everything, you're my home, I love you so much, thank you so much for giving me another chance

To:

It's over, I don't love you anymore. I don't want to be with you.

In the space of 7 days. With no betrayal or anything on my part. I was ready to do anything to help him and save our family after I found out he'd been lying about a hidden addiction for decades. He swears no infidelity and swears he's sober/in recovery.

7 days.

He's cold and detached and cruel now. On dating apps straight away. I don't know this person. I can't make sense of it and my nervous system is shot.

A 20 year marriage over in the blink of an eye.