<human>: My Ex and I broke up nearly a year ago. We'd been fighting a lot and one day after a bad fight I got a text saying he didn't think we should date anymore because he was sick of fighting and didn't trust me anymore, but he still wanted to be friends. I was angry but I hadn't wanted to break up at all and I missed him. He kept trying to talk to me and after a couple weeks we settled into a kind of awkward friendship. He went out on a couple dates with a girl he met a few days after we broke up and it hurt me a lot to hear about her, but things didn't work out between them and the friendship between my ex and I got less awkward/painful with time.

Then, a couple days ago he mentions that girl and tells me she texted him out of nowhere after not speaking at all for a few months. He starts talking about when he first met her. And I realize he actually met her at least a week or two before he broke up with me. I've been upset ever since. I'm paranoid that there are other lies I don't know about. He doesn't know that I know. We've talked since and he noticed that I was being quieter than usual but I didn't explain why. I'm so angry and hurt. We're not dating anymore and it happened a year ago, but I'm still mad. 

I don't know what I should do. I don't know if its worth bringing up something from so long ago, but he and I are good friends and I feel uncomfortable talking to him now. Should I say something? Should I stop being friends with him?

Could you distill the essence of the text into 1-3 sentences?
<bot>: a friend/ex-boyfriend actually started seeing another girl before breaking up with me, lied about it, and I just found out a year later and don't know what to do.