I am not insecure, but I would become that way very swiftly if my boyfriend treated me the way yours treats you. Let’s see. He tells you he does not love you… but he may in the future, if what? If you work for it? Gah.
He tells you there is another girl, a girl who is not you and he says he has loved her very much. Ouch.
He remains in contact with her even though she treated him poorly and you love and appreciate him and when you tell him this bothers you (who in their right mind would it not bother?) he insults you by telling you that you are flawed (insecure) and he (who is elevated) does not have to kowtow to you (who is lowly). Are you getting the picture here? This guy is a jerk! But let me answer your specific questions:
Am I just being too insecure? No.
Will he ever love me and be completely over her? No.
Is their friendship harmless? To you? No.
Once he loves me is he going to love me more than he ever loved her and put her in the back of his mind? No. But this is not your fault. I don’t think he loves you but I don’t think he loves her either. I don’t think a guy like this knows how to love.
Advice? Dump this guy right on his head and resolve to never play second fiddle again when it comes to a man. You’re an Aries! That means you come first or heads should roll.