If you're desperate, you could just tell your GP that you're having issues with chronic nausea. It's not that uncommon, especially if it's episodic. They could potentially write you a script for an anti-emetic drug eg. Zofran (Odansetron).
For me though, I can honestly say the 'Mother's" cures for nausea seem to be pretty effective. Get some crackers, preferably saltines or mayber some graham crackers. Hell, bread would probably suffice. Also, get some ginger ale. The ginger DOES help with nausea. I know it does for me. I'm assuming that your nausea comes in waves when you're on opioids, it does for me and most of my acquaintances. When you feel those waves coming on, grab your drink and crackers/snacks.
When you feel that nausea. Sit in whatever position feels most comfortable. This is a trial and error thing. Some find sitting to be better, others lying flat or at a slight incline. Nibble at a constant basis on your crackers. Chew, taste, use your ginger ale to wash it down in sparing amounts, as liquid will exacerbate your problem. Turn on your TV or music.
You are not nauseous, you are merely listening to music and enjoying some crackers. Nausea is not in your mind at all.
You can't always stop it, sometimes it's just gonna come up. I wish you the best, and be sure to rehydrate with plenty of fluids if you're vomiting. Make sure that you have some kind of balance of sodium/electrolytes as you're rehydrating. It will ultimately be better for your health and probably make you feel better.
Although I practically enjoyed vomiting on opioids once I was rightly accustomed to it. I'd would just stick my head out the car window as if it was nothing and resume conversation, not missing a beat.