My fiance was keeping in touch with his ex and he only told me now, should I still trust him?

Anonymous
The woman, who's the reason why his ex wife divorced him, and who's also married, moved away to end their secret relationship because she got guilty and didn't wanna destroy her family. They were coworkers and she transferred to another location to end their affair. When he told me that story, my impression was everything ended and they are not communicating anymore.

Now, he just brought it up that he was still talking to her as friends because he said he can't say no to people who still needs him as a friend and he didn't bother to tell me because he didn't think it was a big deal.

It is a big deal! This is the woman who he said he loved, and that his ex wife who he had 2 kids with, he never loved. How could he not tell me that? He should have told me at first when he was talking about her, like "by the way, we're still friends and still talking", but no, he didn't mention that!

If my ex reached out to me and tried to be friends with me, I wouldn't hide it from him. I would even ask him if it's okay with him if all my intention was friendship with the ex.

He only mentioned this because I felt like I needed to tell him I was still friends with my ex on FB but we never talk! I even tell him when my co-worker is being flirty asking him if I should block him.

Honestly I can't trust him anymore and I'm thinking of breaking off the engagement.

Any advice?
Updates
1 y
They were talking through email and not in person. I think I should add that.
My fiance was keeping in touch with his ex and he only told me now, should I still trust him?
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