I am 19 and I have been having sex since I was 16 and I still haven't orgasmed?

I've been with my boyfriend since I was 17 and we have been sexually active for 2 years now. When he touches me it feels really good, but whenever it starts to feel really good, for some reason I will start closing my legs and I will start feeling all weird and i'll start shaking, just because it feels so good, but I guess I always just get scared. We have tried lots of things to make it so I won't pull away, the other day I even had him play with my clit and finger me for a super long time, and once I told him I thought I was going to come I had him grab my legs and hold them spread out and start rubbing and fingering me really fast and talk dirty to me(which i find really hot!) and it feels so good and i could seriously have him do it to me forever, but for some reason I will just lay there moaning and trying to just relax and ***, but he did it for like 12 mins and I still didn't! Ah! It's driving me insane, because I am always horny and I never feel satasfied! What should I do?!