Agree with most of the posts and advice such as not wanking for a while before a punt etc but stress and psychology can have a big effect here. If you are going into a punt with it playing on your mind then the problem is probably made worse and you get into a vicious circle, it's then made worse when the girl starts reminding you about the time and that you need to cum soon etc.
Some of the issue could be that you are used to wanking / cumming in a certain way, basically you are in your comfort zone when at home and controlling the situation, going faster / slower as suits you to maximise your feeling and pleasure.
When I punt I like to control the action as I tend to cum too quickly so i start in mish where I can control the pace and once I've overcome the initial urge to come in the first minute I can build up a rhythm.
I then move onto other positions such as doggy and prone bone where i can again control the action, it's rare for me to do cowgirl etc as while it's great when done well most girls don't seem too good at it and just sit on your cock then bounce up and down as hard as they can resulting in me losing my stiffy.
It took me years to cum from oral as again I wasn't in control but this is now my preferred method as I've overcome the psychological issue and OTC gives me an orgasm which is far more intense than through sex.
If lack of sensitivity from using a condom is part of the issue then you could try finishing with an oily tit wank as this gives skin on skin feeling with lots of lube and you can thrust to suit yourself