Girlfriend of a year and a half broke up with me.

I want to keep this as vague as possible because I think she uses Reddit quite a bit. But basically my girlfriend of a year and a half broke up with me out of nowhere and it’s been eating me alive. She didn’t give me a specific reason as to why she essentially gave me. “It’s me, not you”. I do think her older sister was a part of it because I know she was jealous of our relationship because I was treating my girl better than how her own boyfriend is treating her. And her older sister would say a lot of mean things to her all the time, so I wonder if she got into her head to break us up. I am getting on here to see if there’s anyway I can try getting her back? And just general input on people thoughts and what to do now post break up.