It was a love triangle and one scene showed the guy getting slapped because he rejected all the other girls for the heroine. Also, it consisted of a festival scene where the heroine chooses the second guy? Please comment if you know what manga I'm talking about!

I'm sorry but that's literally every love triangle romance manga! Do u remember anything else?

this is probably not it but first manga that popped into my head is Bokura Ga Ita. It's a long shot.