I hope they find happiness… actually, no, never mind
Your ex was someone you once loved more than anyone in the world. You want them to be happy because you know what a great person they can be when they aren't being a cheating POS. You feel kind of sad for them. You know what it's like to hurt and wouldn't want that for them.
Yet, they are also a cheating, lying scumbag who left you for some hot new piece of meat. And lied about it. So in that case, "FUCK YOU."
I'm surprised, but not THAT surprised
I mean, you expected them to break up since their house-of-cards relationship was based on lies. You said as much to all your buddies over that fourth round. But you are surprised. And, admit it, relieved.
Because you were afraid that this relationship would be stronger than the one you had. And because some part of you hoped (even a little bit, admit it) that after everything some lying, sniveling ex put you through, after all the heartbreak and pain, it would at least be for the real thing. Eh -- c'est la vie. What a waste.