RELATIONSHIP HELP!! PLZ

So this is gonna be kinda long but I need some advice. I was in a 9 year relationship which was terrible I felt trapped. I ended up cheating got caught and moved in with this guy. We dated a month and a half he told me he was still in love with his baby's mom and he didn't want to hurt me more than he already did. I was so hurt because I got so close to his daughter and we planned on being together forever. Anyways I moved back home was there for 2 days he wanted me to move back in so I've been living here since we broke up basically just taking care of his daughter. He has another girlfriend she lives a hour and a half away they met on a dating site! So they have been dating since may 2nd. She is moving up here the beginning of august and I got to move out they are looking for another house together. So what I'm getting at is he is still having sex with me a couple times a week and it's usually like right before he sees her or when he comes back from seeing her!!! He is my best friend and like a brother to me as weird as that sounds. His girlfriend is trying to be friends with me and wants to meet I just feel weird knowing I'm still having sex with him. I can't tell her I don't want to risk mine and his relationship. I still love him to death and think if I hang around and prove myself he will want me back one day and I know if I'm in a relationship with someone else I'll leave that guy for him. This sucks so bad he broke up with me in April and I'm still heartbroken.