Well, your best bet is to masturbate before you have sex with your fiance. This way, you wont finish as quickly. Also, practice makes perfect! So, practice with your fiance ... alot. good luck to you!
Dry hand-masturbation trains your brain to ejaculate earlier.Use lubrication. Do not masturbate when your are over-excited.Do not visualize intensely. If you are in a hurry do not masturbate. Keep enough time for masturbation. Jerking along with thrusting is important as it simulates real sexual situation.Do not stop breathing.
You may try withhold and continue method. when you are on the verge of ejaculation,stop masturbation for few seconds and then continue without ejaculation. this is one round, do such rounds three times. In this way you masturbate three or four times and ejaculate only once. This method trains your brain to delay ejaculation.
You may googlee Satkriya, kegel exercise, deer exercise to enhance your sexual stamina.