Why am I so insecure with our relationship?

So me and him have been dating for six months. Seven on the 19th of july. And I'm still insecure about him looking or commenting on other girls.... And when he talks to his exs I still have trouble with that. And I see there conversations. He doesn't hide it from me. But he's my first long term boyfriend. And I'm new to the whole dating thing in general. In 19 and he's 22. I just don't understand why in insecure about it. Is it just cuz I love him so much I don't want to loose him? I trust him with my life. Any suggestions on what I can do to make myself feel better. And him.?