If you are used to only getting off by masturbating, then it does take some time to get used to cumming by another method. If your partner is willing to work with you I think you will eventually be able to relax enough and enjoy the different sensations enough to orgasm.

Also, you didn't mention your penis size. Using a condom for sex makes it harder to cum especially if it doesn't fit well. Try different sizes or types if you haven't already.