Hi Discreet, OK just stay calm, I want you to go and google dobson and ross, and on here you can read and learn just how to give oral and how to extend your foreplay, and lots more.
Now the next thing could be that your thinking ahead of what your about to do,now most men would just lose there load,pre-ejectulation, then game over, but you just lose you erection, things behind this is performence anxiety, or in your spare time your over masturbating, now if that is the truth then give it a break, now there is a way out of this, and it would reboot your body, its a total 8 week break from all forms of masturbating and any sexual contact for the 8 weeks, so with this gorgeous girl will just have to be put on hold, and time to be straight with her if you really love her, and she you, tell her your problem and say its just 8 weeks, then we can be at life two rabbits.
Come back to if this fits the your and what you have been doing.
Good Luck
in your spare time, do you masturbate a lot or watch porn a lot?
I would go get these supplements: L-argimine, Maca, and ginkgo biloba.
Start running or other aerobic exercises and work your large muscles with squats and weights. Don't get injured. Just get a nice workout and do some stretches afterward to get your blood really flowing and the relaxation to come on. A relationship and sex should be fun.