There is a learning curve for you to solve this problem.
You have to train your visual response to her beautiful, naked body and genital.
When you touch and see her naked body, you must have a calm heart and regular blood pressure, that means, you must be in a parasympathetic state to promote erection. If your heart is racing and your blood pressure is exploding, you can not perform, resulting either ejaculation or erection withdrawal. Your sympathetic nervous system is in charge of your body function, executing "Fight or Flight" interpretted as "Ejaculation or erection Withdrawal" in sexual function.
How to get used to her body?
1. Frequently have naked plays with her without having sex - training your visual response.
2. Frequently Use your penis to massage her libia and clitoris without penetration ( don't let her hands or mouth do your penis!)- training your penis and your mind.
3. Practice Screwing intercourse in every morning for 10-20 minutes without forplaying your penis (you have to play her libia and clitoris for lubrication) if possible; After penetrating her, calm you mind first, don't pump, and screw her with slow motion without contracting your PC (prostate) muscle; if necessay, have a deep breathing, exert a pressure against your bladder to relax your prostate muscle, and contract the tailbone muscle between the tail bone and anus; Don't ejaculate!; after screwing jump out of bed and go to work.
4. During lovemaking, repeat Item 1, 2 and 3 for a rehearsal before starting your or her pumping. Then following the lovemaking procedures as described in
http://www.linplaza.com/cases/case7522.htm
http://www.linplaza.com/love/fgplier.htm Also see the attached pictures.