Pleas help

- We just came back from a date and had been drinking quite a bit the whole night . My boyfriend of 8 months went to the restroom and left his found on the nightstand . Yesss I was snooping . I went through his texts I went through his pictures and saw the hidden file on iPhone . Then I saw a video of him naked with some girl . It certainly doesn’t look like me and caught my attention. I press play and it was him naked going to penetrate some girl . My heart completely dropped . I was so shocked that I didn’t get to look at the date of this video . I yell his name . I ask him why would you cheat on me and I slap him . He then slapped me back . ( I know I shouldn’t have and I later apologized to him my emotions got the best of me ) he then started explaining that he was in a relationship with a girl years ago 2016 . And that he had completely forget that it was even there . I called my fiend to pick me up and drive me home since I was drunk . My boyfriend came over to my place and repeated himself over over that he forgot it was on his phone . For the past two days I want to believe him but I have the image stuck in my head all day long and I tell my myself he forgot about it then I tell myself not to be stupid how do you forget something like that on your phone for almost three years let alone for the past 8 months we been dating . I have emotional breakdowns all day. I still can’t stop shaking and I haven’t forgotten about this image once .

What do you guys think ?

Is it possible to forget something like on your phone for two years or the past eight months we have been dating ?