Ability to orgasm?

I've been with my boyfriend for a year and a half we've been sexually active for about a year now and I've never had an orgasm. Sex feels good for me but I'm never able to finish. I've tried so many different positions and different things but nothing does it. I feel like I take too long so I never keep on trying that long. He is always willing to keep trying and wants me to finish but I always end up getting frustrated and I don't know what else to do . Help!