I've been sexually active since i was 16, I'm now 19, ive had a lot of sex in that time and only just 3ish months ago did i have my first orgasm, for some its just not easy, people have different physiologies which is party to blame apparently, its not your fault.
how adventurous are you in bed? i used to be really scared of being on top, absolutely terrified, but eventually i got into it and eventually got good, and when that happened i started to be able to orgasm, so you could try being on top, it gives more friction with the clitoris. I've also found that sex in the morning is really good for orgasms, apparently there's an increase in testosterone, in both men and women, and it increases the ability to orgasm. most importantly though i think is that you just need to stop worrying about it, let it happen to you, worry wont help anything and is the biggest hindrance in the bedroom