My Childhood Friend Girlfriend Was C**kolded by the Protagonist! Well, I’ve Known This for a Long Time

This world was a shitty game. And I was the side character who got cuckolded by the protagonist. So I moved according to the setting, not against the game, and finally, the protagonist cuckolded my childhood friend girlfriend. Now I could freely fall in love. I thought. But it seems that I have a problem with my ex-girlfriend … This is the story of me, a supporting character, surpassing the protagonist.