When having sex I ejackulate quickly?

Several remedies. Certain antidepressants (ssri's like prozac, zoloft, lexapro, paxil, (paroxetine) and snri's like Cymbalta and effexor) have been shown to delay the time to ejaculation in some men. In addition, there are anesthetic creams and condoms that can be applied to the penis to decrease sensitivity, that also work fairly well.
Premature ejaculatio. Premature ejaculation is common condition and can be defined as present if the time from the begging of intercourse to completion is less than 3 minutes. The average period is reported to be about 3 to 5 minutes. Serotonin reuptake inhibitors have been reported to be effective in increasing the time in takes to ejaculate.
Practice. The simplest answer is to have sex more often. Each ejaculation will delay the next one (the same day).
Premature ejaculatio. Possible some topical agents could help (as long as it doesn't affect your partner. There are also certain oral medications that can be prescribed to deter pre-mature ejaculation. Either way, a urologist should be able to help you.
See Below. Sound like possible premature ejaculation, rule out any possible medical issues, if not medical. Sex therapist maybe of assistance to find out why. There are masterbation techniques you can use to train yourself to prolong activity and some of the ssri's especially zoloft (sertraline) has been used for premature ejaculation.