Boyfriend's best friend is a girl - How do I get over jealousy?

My boyfriend (of about 3 months) has a best friend who is a girl. They have been friends for a long time and they tell each other “I love you” and send hearts and stuff – she lives back in his home town. He says he doesn’t love her in a romantic way and obviously doesn’t want to ‘date’ her, and he claims he doesn’t want anyone but me, but the hearts and i love yous make me really insecure. How can I get over this? I don’t want to be a crazy jealous girlfriend.