Mesh gear is a great option but only in certain kind of heat. In an environment where you have dry heat and the air is really hot, it's actually better to wear gear with less ventilation, otherwise you're just getting hit with hot air and your body doesn't have the opportunity to maintain its own temperature.
As previously mentioned, summer base layers are an essential piece of under armor to help you deal with the heat. You may also consider looking into getting a cooling vest. I used one on a trip through Death Valley last year and it works surprisingly well!
http://www.mototude.com/REV-IT-OXYGE...t-p/ftu104.htm
http://www.mototude.com/REV-IT-OXYGE...t-p/ftu204.htm
You also gotta make sure you have a helmet that offers great airflow and ventilation. A poorly ventilated helmet will cause you to have some of the symptoms you mentioned, i.e. extreme discomfort and nausea.
I hope you find something that works well for you! I know the hot weather can be extremely difficult to deal with.