Boy! I am a sex genius. Now I have some questions for you:
1. How old are you?
2. How often do you ejaculate?
3. When did you start your masturbation?
I need these information in order to help you out!

Your problem is caused by sexual exhaustion. I experienced the similar problem when I was young. It is usually caused by multiple ejcaulations in one intercourse session in an attempt to make her come.
Here is my advice:
1. You have to take a herbal product like LoveLonger for 30 days; please go to: http://www.linplaza.com/cgibin/herbform.htm
2. You should reduce the ejaculation frequency so that you don't feel exhausted after ejaculating; You have to do it;
3. Practice the Sexual ChiKong Breathing to control your sexual stimulation so that you can build up your sexual energy during intercourse. You have to practice ejaculation control to make her come without ejaculation. The most trick step in addition to the breathing technique is the massage of your penis base and footing every time when your penis is out of her vagina. When you are about to come, hold it there, have a deep breathing and pull it out slowly. Then Massage the penis base and footing. This will not only increase your blood/bioelectricity(Chi) into your penis, but also reduce the sexual stimulation on the penis shaft. Alternate the love position and sexual driver when your trigger zone is overstimulated. This is my penis power-up method. Don't let her suck your penis, in order to prevent the overstimulation on your penis' trigger zone. Instead, use her vagina to power up your penis. Practice srewing technique - a slow motion to start your intercourse sesseion.
http://www.linplaza.com/love/screw.htm
4. You should use the srewing technique to have a therapeutic intercourse without ejaculation for 10 minutes everyday. This is the most important therapy for curing the sexual problem. Do it every morning.