Am I just worrying over nothing?

My boyfriend tends to like to hang out with others girls...I wouldn't mind if he went out with them in a group...but he chooses to go out one on one..and that bothers me. I told him that I don't like him hanging out with other girls alone and he just says, "you have to trust me if this relationship it going to work"...so then I feel horrible for not trusting him...but it still really bugs me. Not to mention that the girl he likes to hang out with has a crush on him and he knows it...she openly admitted it. So I don't really know what to say or do anymore. Am I just worrying over nothing? Am I being an over bitchy girlfriend. Or do I have every right to be upset by this?