Narcissistic ex

Ex of 6 years cheated on me again and days after he started a new relationship with someone new. They we’re together for a month but then he texted me saying that he was unhappy and was thinking about me the whole time, he even sent me screenshots of messages he was having with his friends days/weeks prior to him reaching out to me in where he was telling them how he misses me, that he fucked up and that he was going to leave his current girlfriend because he didn’t feel anything for her and didn’t want to string her along. He left her but I decided to let the girl know that he was telling me all of these things behind her back. He was claiming that he still loves me and will always love me and that he wishes he wouldn’t have done that what he did. But how is it possible that now I come to find out that he went back to her days after telling me all of this?😭 I feel stupid for believing what he was saying to me.