Help! this girl is messing with my relationship! How do i fix this?

Okay, so heres the story (stay with me here) 6 months ago my boyfriend and I broke up for a while and he kissed or "made out" with this annoying girl who he's been friends with for a while (she happens to be a skank also, excuse the term ha) But anyway, we got back together, he lied to me about it and i just recently found out. Now i feel very insecure. Also just recently he went to a party and she apparently kept trying to talk to him and leaned her head on his shoulder. He moved away but its just the fact this girl has no respect for our relationship and we've been together 2 years! He agreed to tell her to stop flirting with him so he messaged her and said she should stop doing stuff like that since they are "just friends" but i know they talk still and im sure she's flirting with him as usual and i just dont know If im overreacting or what. We are going to different schools this year and im very nervous. help!