Don't have sex for a while including masturbating. Stop worrying about it and just focus upon her. Relax and let her do the 'driving' this time.

Surprisingly, reducing foreplay can help. Much of foreplay with hands, mouth, etc, is more vigourous and stimulating than the vaginal environment - which is quite gentle. To go from vigourous stimulation of the penis to the more gentle is like "backing up" in building the tension needed for ejaculation.