Boyfriend

I'm not really sure how this works but I need some advice..... my boyfriend broke up with me tonight because people told him I cheated. I would never cheat. He believed it Bc today when we went to the lake he wouldn't speak to me or even look at me Bc his family was there, so I swam with my guy cousin and his girlfriend so he thinks I was flirting with my cousin so I'm apparently a cheater?? I've been crying my eyes out since 7:30 pm at the lake when he wouldn't even acknowledge me to now where he's left.... I don't even know what to do... I just wanna fix everything but he's believed lies 😞