I am obsessed with his ex-wife- why?

I have been with my fiance for over 3 years now. his ex wife and him have a 4 year old son together and have 50-50 custody so we see her at least 3 times a week and she calls\s nightly (or we call her) so that their son can talk to them and tell them about his day. For some reason i have such a need to out better her, impress her..everything and i am constantly comparing myself to her and i dont know why. i know that my fiance loves me more than anything and i certianly dont want to replace her as a mother because she is a good one, so what is driving me to be like this? WHat can i dot to stop?