Why does it hurt more now?

So discovered my ex has moved on really quickly. He literally hooked up with some woman a month after he left.

However I didn't find out til this past weekend.
He'd been texting me saying he wanted me back making out how he's lonely, and guilt tripping me. The whole while meeting with this new woman.

Now I don't want him back, he hurt me too much was deceitful and couldn't deal with the important things.

So why does it still hurt? Is it because I wasn't worth the truth or because he opened up old wounds of how he was whilst we were together?