Im very Insecure in my relationship...?

I've been with my partner almost 18months.. We get on great, are starting to plan moving in together in the next couple of months and things are going well.. He has two children, and still see's the mother of his daughter.. They text sometimes, but only when it's to do with their child, and they have an amicable relationship.. I know that they only still communicate because of the daughter, but I can't help getting a knot in my stomach about it.. I feel very insecure about it and any other women. I get scared one day he'll think I'm not enough. Help..!?