Seeing your ex with a new partner?

Seeing your ex with a new partner?
how does it make you feel?
was with my ex for 9/10 months, lived together, got a dog etc, I truly loved him. he cheated on me a couple of times ( that I know of) one with his the ex before me, and one with the girl he's now with...8ish months later.
shes now moved down to where he lives (same town as I live) so theyre obviously living together...i saw them out the other day together...but for some reason didn't get the sadness I thought id get. it felt weird seeing him together and I felt a bit bitter that they were STILL together (he cheated on this girlfriend with me at the start of their relationship a few times)...i didn't think I was over him cos I still think about him...but seeing as I didn't get the sad feeling when I saw them, maybe I am?!
any stories? how do you feel when you're in the same situation? x