I can only get aroused by imagining someone else but not my wife

I am married for a couple of weeks now and I am not able to get aroused by her. I have to imagine somebody else all the time. Before marrying her I used to have sex very often with different woman. I never told my wife who I been knowing for the past 3 years. We never had sex before marriage and now I am unable to have sex with her often. I seem to be making some excuse or the other. How can I get over this?