disrespectful boyfriend

Last year i was on my bfs phone and this girl was texting flirty stuff. I trust him but told him i dont like how she talks to him. A little later i saw he was flirting back and told him i didnt like it. He said okay he wont. I changed her name to the girl i dont like to casually drop a hint. Well who is texting him now still this damn girl. I trust him with many girls who are friends but i cannot stand this girl. I feel so disrespected because he know exactly how i feel. I would never do this to him and dont know why i deserve this. Last summer he told me not to have lunch with this guy because he didnt like him and i listened. Im trying not to be psycho it just hurts me that he would continue to do this. We have been dating for 2.5 years and this is the one thing we constantly argue about. I just dont get how he can say he loves me and text her too. (She flirts and idk if hes oblivious but does it back, ive never met her so idk how she is in person. She wants him to go to a party next week and i havent said anything because honestly its his fault of something happens and not worth upsetting me)