No idea, sorry.
I have to do the same thing (but I don't orgasm during sex anyway).

you know what? I think i hold my breath just before i cum too, but i dont hold it the whole time. it might just be a habit that you need to break out of. Try masterbating without holding your breath. no matter what, do not hold your breath. You may cum finally or you could be doing it for an hour and still not cum. It may not work out well immediately but if you keep trying sooner or later, you WILL cum. Its not gonna break in one day, you have to keep it going ever so often, kinda like an addiction you have to quit. The body is very smart and has a tendency to naturally stick to habits so just keep trying until your body gets used to cumming without holding your breath. its worth a try.

This is a VERY bad habit as this sort of tenseness will prevent you from having an orgasm in the first place. You're now having to fight for that one orgasm instead of enjoying all that you can.

First step: stop working so hard. Sex is fun and orgasms are both enjoyable and good for you. Often, a good mental attitude toward sex will improve the quality of sex. This is not the Olympics - this is playtime!

Next: Pay attention and deliberately begin deep breathing as soon as you notice you're holding your breath. Feel the air moving in and out. Notice how it relaxes your body and heightens the sensations you're feeling. Go with it. Have your lover watch for you if necessary so he/she can whisper "Breathe" in your ear at the right time.

Finally: After the orgasm, do not stop. Continue the stimulation and deep breathing. Relax even further and move through the sensitive period. You'll then have another orgasm. And another. And so on.