Loss of erection during sex

Hi,
For the past year I have been having problems maintaining an erection during sex with my wife. If I masturbate I have no problem maintaining it.
I think the problem started when we started to try for a child and the pressure of the try at the drop of a hat because the time was right threw me off. The problem has become so bad right now that we have only had sex twice in the past 7 months and I have given up half way through because I lose my erection.
This is causing a strain on our relationship,I am attracted to my wife its just that the sensation when I have sex is not present. I do not feel much pleasure while thrusting and then I start to think I am going to lose it and straight away I do. Nothing works getting my erection back.
We are also not that into foreplay which is also a problem and now both of us dont want to try because we know what will happen.
Is there something I can do, take? Any help would be appreciated
Thanks