I have been cheated by girlfriend and cousin, why was this betrayal by family members

Namaste sir, I’m Prateek, DOB 13 Jan 1991, born at 2:30 AM in Lucknow. I found out that my girlfriend Neha (DOB 6 March 1992) cheated on me with my cousin. She said she was emotionally confused. I feel humiliated and broken. Why did this betrayal happen within my own family?