Am i going mad?

iv been with my boyfriend a year and a half who has an ex the ex dont like me she doesnt no me but because im with him now she dont like me, any way she is 5 years younger then me and some of the things my boyfriend has told me about her is that she desergreed with every think he said and put him down.. i feel like when he askes me a question and i answer him he thinks im disergreing with him like how she would, but im really not, when we get into an arument i tell him im a women and stop talking to me like im your ex, i cant stop thinking he wants to be with her aswell, shes blond and iv got brown hair and i sometimes think he wants her back please help i cant get it out of my head and why do i always bring the ex girlfriend into out arguments, also he says he thinks im disergreing with him cause thats how he was brought up by his mum and dad they always put him down. but he was with his ex a few years so i just think he thinks im like his ex ahhhhh