Can't orgasm! Help!

Okay so when me and my man get intimate he can make me orgasm while going down on me but NOT when we have sex! 
He's is so hot and I love him so much! But we have been together for 3 months and not once have I orgasmed while having sex! 
I have uhh faked it so he knows I don't always finish but doesn't know that I have never finished!! 
Please help! Are there any tips on how to orgasm during sex?!