When Im without my best friend I get insecure and think he doesnt care. How do I change?

I've always had issues with insecurity and I know it's down to one of my parents not being there when I was growing up. Now I'm growing up and I form attachments quickly but I always lose people because I'm so insecure and it makes me get angry at them because I think that I'm always second best. In the last 6 months I've met an amazing person, he's my best friend and I trust him with my biggest secrets. My problem is that I need constant reurance, I need to talk to him all the time and I worry that one day my insecurities and then my anger because I think he doesn't care, will end our friendship because it overtakes everything else. How do I change who I am?