I have been good to my girlfriend of 4 years and she just randomly left me for this other guy, what should i do?

i have been with this girl for 4 years and it has been a wonderful 4 years, anyways last week she lied to me about going to stay with a friend when she was really going to see this other guy. we weren’t fighting and she seemed happy with me at the time. i called her that night and she accidentally answered her phone and didn’t know that i could hear what she was saying. she was talking to that guy about breaking up with me for him, i don’t know what to do. i have tried talking to her and all she does is lie to me about being with someone else and she is living with this guy now. i am just so heartbroken over this, i really wish she still felt for me like she used to, i am 22 btw.