My girlfriends always cheats

So like... Thats not the first time its happening, it happened before but i forgave her because im stupid and so like we were happy together for long time but now like 3 days ago she met a guy and the guy became her new boyfriend i mean my girlfriend says hes just a friend but when i asked the guy if hes dating my gf he said "sim" ...i dont know what that means -_- but it translated to "yes" so uhm im very nervous and scared now... I don't want her to cheat again i don't know what am i doing wrong like what am i supposed to do to make her stop i just wanna be really loved by someone