Gina, unfortunately it would seem that your service manager doesn't care about you. That doesn't mean GM doesn't care about what's going on. I can imagine how frustrated you are, but as others have suggested you need to switch things up a little, try another dealership or talk to someone else at your dealership. If that fails start a Lemon Law claim process. Call GM directly, but don't expect to talk to the CEO.GENERAL MOTORS DOES NOT CARE ABOUT MY HEALTH
There's a Gm Customer Service rep on this board named Stacy, get her involved, give her your dealership info and plead your case with her.
In the short term try to identify what things seems to make the problem with your car worse. If you're always driving at really high speeds when this happens, try slowing down and/or taking a different route. Try turning the recirculation function from "frsh air" to "recirculated air" and open a window. Try different things to keep the car from venting fumes into the cabin. If nothing works stop driving the car! It's not worth your health.
Good luck and keep us posted!