How can I trust him?

I have been with my fiance for 2 years now.

But last November I saw that he had been sexting his ex, no mention of me and he says to her that he was glad he lost his virginity to her! Then he said that she could have been his fiancee if she didn't dump him, obviously trying to make her regret dumping him... This deeply upset me.

So I confronted him about it. He said he had no feelings for her and was just trying to mess with her (he was the one mainly complimenting her and she just said "thanks." or "ok" to him) and he cut ties with her, blocked her on everything and I felt a little more comfortable but after 4 months I can't come around to trusting him fully.

How do I start to trust him again?
How can I trust him?
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