Advice?

Ok I have horrible trust issues my current bf has left me high n dry once before like 6 months ago to go with this other girl and I took him back n forgave him after that I'm still with him to this day but he continues to hang around this girl n I hate it when I'm not around them because I think the absolute worse thing ..but my boyfriend tells me I'm the only girl what should I do take his word for it ? Or keep assuming he's trying to hook up with both of us at once? I need some answers :/

Thank u