I have a girlfriend of 2 years who I love, but people say she is a horrible person?

People keep saying my girlfriend of 2 years is a horrible person, a bad person etc. I can see that she is distraught when she is confronted by people who say these things, but she keeps lying to me. I have already cut off my best friend for her because I love her. He and my other friends said she is a horrible person and she then gave me an ultimatum- she asked me to drop him as a friend or I’ll lose her. She was crying while she said this. She said my best friend was just jealous of our closeness and our relationship so that's why he doesn't like her. So now I don't speak to my best friend or any of my friends as she doesn't get along with them.

My girlfriend has lost two friends though ever since, and they say she’s not been a good friend to them and is a horrible person/"bully". They (her ex friends) have reached out to me to tell me she has been sneaking around behind my back. But my girlfriend assures me they are the ones lying.

Who should I believe? I love my girlfriend and want to trust her, I do trust her, but it’s weird how many people are saying the same thing to me. I have caught her lying to me before.