I am in terrible emotional pain...plzzz help?

hi there, plz listen to my problem and tell me what to do, i am a 25 years old guy and there is this 22 years old girl i know and we went out a few times and i like her and one day i introduced her to my 17 years old cousin and so...the three of us all started to hang out and have fun and....i told her that i like her but unfortunately....she said she doesn't feel any connection with me and she just likes me as a friend but....this is not the real problem, the real problem is that she told me that she likes my 17 years old cousin and she is dating him now....i really got heart broken and cried alot...what drives me crazy is that how can she date a guy younger than her by 5 years??? me and her are close in age but she rejected me and took my cousin who is 5 years younger than her, i regret the moment that i introduced my cousin to her, now i feel pain everyday and can't stand that she and my cousin are dating while iam left all alone