Your anticipation level is peaked !! You're in over stimulation. As said previously, rub one out earlier in the day and during foreplay, have her stay away from that tingly area (medical term huh) just below your cock-head.
After 23 years of marriage, I find myself blasting off too soon at times. My wife loves to rub me just below the head, but realizes it dooms me to cum all too soon.
She also squeezes the head of my cock, gently, folding her hand over the ridge, forcing blood down the shaft. That will quelch things pretty good.
Have your girl play with your balls and scrotum more, and try to relax and enjoy the moment. For soon you may find yourself in marital bliss (?) and not feel someone elses hand on your goods for weeks on end.