Why am I feeling this feeling? I feel I am losing it.

So the last time I posted her I talked about the chick that I broke up with, and honestly I didn't feel much when I did it, in fact I felt great because she was a horrible person.
But I just discovered that I was right about her dating the guy we broke up over. She was dating this guy long distance while also dating me, she hid the fact from both of us (The poor guy is still doesn't know), and now that I know I was right and how she gaslighted me makes me feel betrayed and mad. Why am I feeling this when she's an ex? I should be indifferent about her.
Seriously, I know know how to feel about all this. I feel cheated and used, and the fact the guy was a downgrade doesn't help with my ego either (I know ego's bad, but I am human).
I feel I am losing myself, losing my composure, I feel I am becoming petty and unable to completely move on. Also, I don't understand why she keeps looking at me and smiling (I think she smirks at me)

She's not only cheated on me, she's still cheating on this guy. While he's taking her out, spending time with her, listening to her problems and giving her a shoulder to cry on, he's oblivious to the fact that she slept with me so many times, I don't think any guy would like that, however pathetic.

I just want to close this chapter and feel normal.