Is karma a real thing?

my(22M) gf(23F) left me for some another person.the thing is she cheated once before but i was so in love with her that i thought she would change.now she's with another person and that person showed me a screenshot in which she said " i am not the person anymore like before, i really love you and want a future with you". it just happened a month ago and she's thinking about all this with that guy. the way we broke up she was talking to him at that point and when i found out about this she said he treats me better. i was so into her did everything for her and she just left me for a guy who was making her feel better since some days. people say that they realise what they lost after doing all this but is it really true?does karma really hit back to such people with such incidents? Edit: its only been 10 mins but i understand what i was thinking is purely wrong. everyone deserves to be happy, i shouldn't be thinking negative for that person, god bless everyone.