friends but not really friends

So my one friend that I have has been hanging out with this girl who is also one of her best friends but she is always rude to me and is disrespectful every time I'm with my one friend and she treats me like I'm stupid and when they're together they both rub off on each other and then my one friend starts to treat me like shit and I don't know what to do because I don't want to lose my only friend but it feels like I already have and I have no one to turn to.… please comment some advise, I could really use some atm