Lying... How can you get someone to come clean and be honest?

I am 95% sure my ex dumped me for another girl. Before the break up I had seen a couple messages between him and another girl on facebook. I got suspicious so when he wasn't home I went through his message history to find out more. I saw that they had gone to a few clubs together and he had been to her place, he had told her he sleeps naked. There was a message inviting him to go to volleyball that evening so that night when he went I went with him and it was really awkward I could see that she was waiting around for him and he was ignoring her. Then the next day he got a message from her saying she was confused about his behavior and that was why she was so reserved the night before.

When we broke up I asked about her and he said she was just a friend from uni and that he never saw her outside of class. I knew he was lying and that he would never tell me the truth. So I asked for distance and tried to move on. Now he is back in my life and wanting always to have sex with me. And a couple days ago I stumbled across a photo of him and this girl together at the uni ball (a month ago). I decided to come clean about reading his messages in hopes he would follow my honesty and tell me the truthl. He didn't and the next day I asked him if he had gone on any dates with anyone and he said no. Then I told him about the photo and he said he went to the ball alone and that she was just standing nearby so they got a photo taken together. I asked him about other times I knew they were together and he always tried to deny it but I told him I saw messages that prove otherwise. I caught him in many lies but he was still claiming that they were never together. I also found out he gave her a plant for her birthday (and doesn't giving a living gift signify something?)

Anyway, I asked why he had lied to me before about her when I didn't even know about her. He said if he said he told me about spending time with her I would have been upset. But he has many female friends and I don't care if he spends time with them so really why would he have to lie about seeing a girl who is just a friend? Does anyone else think he is obviously lying to me? And he knows I saw all their flirty messages, so if he is lying why does he still think he can get away with it when he knows I've seen the messages?
Lying... How can you get someone to come clean and be honest?
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