sexual past

i can’t get over my boyfriends sexual past. we have been together 9 months and i think about it every day. we are in high school, and where we live (utah) sex isnt too common in high school. there’s a few people but not many. i’ve never done anything sexual with anyone but him. but before me he had a girlfriend and they did oral is all but it haunts me every day. i’m not holding onto the past i forgive him and believe that he loves me but i can’t get the image out of my mind that SHE saw him naked. he saw HER. they were in those moments and felt that together for the first time. and it wasn’t just for pleasure, they were together for awhile and he says he loved her. which makes it even worse. HELP A SIS OUT how can i forget about his past and not let it make me so sad every day