Liar liar liar

Ex was dating a girl for a month but he reached out saying that he’s been unhappy, he thinks about me when he was with her and he showed me conversations of days prior, when he was also telling his friends all of this. He even showed me conversations with her when he was acting like an asshole towards her. He dumped her and after I decided to meet up with him, we slept together, hanged out and then said he can’t do it, that he should let me be happy but it was all an excuse because now I found out that he went back to that girl. Idk why he decided to go back to her if he was claiming that he was not over me. He was telling me all these things and now he’s back with her. Any advice ?