How can I forgive?

hey guys, my “bf” went out to eat with a group of his guy friends. while they were out, two girls sat right next to him and they started talking. they followed each other on instagram and he started sending one of them memes. i noticed him following them on instagram so i asked who they were and he told me he had no idea who they were. one day i was scrolling on instagram and they went out to eat again and one of them posted a picture of all three of them. i brought it to his attention and he finally admitted that he knew who they were. i msged the girl the night i found out and she sent me screen shots of their conversation, it was memes. she said i had nothing to worry about because it was just memes. I don't know how i can get over this.
How can I forgive?
Post Opinion