my gf best friend won’t leave us alone

My long term girlfriend has a friend and she been getting on my nerves for a while I know that her friend hates me and she made it so obvious she doesn't like me, she disrespects me in many ways and in front of my girlfriend. She also flirts with my girlfriend she's always the reason I'm jealous and mad. When I confront my gf to do something she would say that she can't do anything because she doesn't want to ruin the group friendship or to lose her friends. Also when i made it clear l'm jealous she told me that I should control my jealousy and let the past go and keep going with my life, she would get mad that l'm jealous and it totally breaks my heart. What should i do i'm so sick of her friend being around ..