I think he cares about his best friend more than me?

Ok I've been seeing this guy for a month, and I've never been happier than when I'm with him. Only one problem, his best friend is a whiny attention seeking girl whose name he has tattooed on a part of his body. Before we started dating he told me that he was once in love with her but she never returned the feelings. Since he's said that, I can't believe that he's not still in love with her. It hurts me and I don't feel that I measure up to her, even if he tells me I'm the most important person in his life, I can't help thinking that I'm not his number 1. And when I discuss it with him all he can say is that he won't let her out of his life because they've been through too much. I don't want to be jealous, I just want to know that this relationship is safe. What should I do?