Please Help Me ?????

Hi again girls , please read & hear me out , so remember when I told you girls about my boy bf ? Yea well there's a little more to it ... his ex gf / fl or w.e she's supposed to be is still in the picture & they act like a couple but they aren't together , they walk the halls together , & she's over his house more than she goes to school , & it really irritates me . He tells me it's nothing but I think that's it's something ... hints to why I'm feeling this way ( I need to stop having sex with him ) , please help me & tell me your thoughts