First of all, as long as your guy loves you then you got nothing to worry about. Try to tell yourself that they are just friends and nothing else. I know a few people who are still friends even after the breakup. Your boyfriend's ex might had realized that they couldn't be boyfriend+girlfriend so they are better being friends.
From my personal experince, I was never insecure with my boyfriend having tons of female friends and I still am not insecure. His ex showing up did shocked me a little bit and made me insecure but I kept telling myself "nothing to worry about, he loves me and he is faithful" so try telling yourself the samething. Believe me, once you start trusting him, the better you will feel.
Never make illusions of your boyfriend cheating on you, believe me guys hate that. Try to stay tranquil about this situation and keep reminding yourself of how much he loves you. believe me, when a guy loves you, he will keep telling you those 3 special words on random and when he tells you that he misses you, he seriously means it. Don't limit him things. It is a good idea for your boyfriend to hang out in groups of girls instead of a girl alone because people spread fake gossips around. Don't mind him talking to his female friends alone on the phone because that doesn't mean anything.
As long as he remains faithful towards you matters. Notice his behaviour patterns though.
Best of Luck