I was having sex for 2 years before i finally had the big 'O' I was in the exact same situation you are in, i felt very frustrated. I ended up talking to my gyno. about it. And she suggested that i just focus on relaxing and try using a vibrator during sex. (i have the best gyno. i know)
I followed her advice and i started masturbating more, before it would take me a really long time to get to orgasm, now it only takes 5-10 minutes . . . my gyno. explained that the more you masturbate, the more you are 'training' your body and brain to reach full orgasm. Often, i will use a vibrator on my clit when i'm having sex, esp. if he's penetrating me from behind and then i can manage simultaneous orgasms, which has been really great.
So in short, this is a normal issue that allot of women have and may take some time, (i know it did for me) . . . i would suggest you:
~ Get some sex toys/vibes
~ try using sex toys while you're having sex
~masturbate more
~ relax and just let it happen
remember, the more you do it, the faster you will train your body and brain to reach the full orgasm
good luck! and we're here for you!!
Sweetpea