That does sound quite frustrating, although not uncommon. I actually hear this more frequently from people than you might think, and it isn’t just from women. More and more, people are complaining about the inability to “finish” in the typical sense.
You say you orgasm fine from masturbation, so you know how to get yourself off. How is the quality of partnered sex you are having? If everything else is good, have you shared your technique for orgasm with your sexual partner? Perhaps there is something that you do for yourself that your partner is not aware of, and they just need a little more information about how to help get you there. If the quality of the sex you're having is not so good, you may need to have a bigger discussion with your partner - or find a new partner altogether.
Another thought that comes to mind is how present are you mentally during sex? As I work with more people, I noticed that we are so busy mentally that we are actually not even participating in sex so much as going through the motions while the mind has already moved onto the next thing. So, I challenge you to try to fully participate - both mentally and physically - in the act of sex. Are you actually experiencing the sensations going on with you and around you or have you mentally wandered off elsewhere? Being present can really help get those orgasms going again.
Also look at setting yourself up for success in terms of minimizing the ability for distractions to happen during sex. Mute your phone for a bit. Turn off the TV, or put on some good music. If there was a bunch of stuff on your mind, jot some notes down so you don’t worry in the middle of sex that you might forget something. Or, consider (if you can) changing the time of day you are having sex so that those distractions aren’t there or as prominent. If you use a toy, bring the toy to partnered sex. Most women don’t climax from intercourse alone.
Finally, try not to focus on that orgasm too much. Relax a bit. Be patient. Allow for the orgasm to happen, and allow for yourself to just enjoy the moment without the orgasm from time to time. I wish you luck!
