I've struggled with this immensely. Everytime I have had sex I haven't been able to maintain an erection. I've probably only ever had penetration sex less than 10 times and every time i can remember i've lost my erection and never been able to come.
In lieu of this issue I have felt a need to avoid sex and intimate relationships and it sucks. All I can say is try to relax and talk with your wife about it, you might be suprised at what could come from being completely honest with your wife about what your going through and why you think you have this issue, and go easy on yourself, cheers