Common houseflies are attracted to decomposition of meat and sugar. People who find themselves the specific targets of flies often smell sweet to flies usually because they use some kind scented hygiene products. Diabetics in ketosis can also attract them in theory.
To my knowledge, flies do not drink directly. Many insects have trouble drinking water because at their scale, the surface tension of water can entrap them. In in dry areas, you seldom see insects drawn to fresh water. More often, you see them drawn to some liquid with lots of biomatter in it e.g. cow urine. It looks like they're drinking but they're really trying to eat. Plus, the biomatter lowers the dangerous surface tension.
Insect get their moisture from food and the air. If you find flies targeting your sweat specifically, then its likely they perceive it as sweet or containing ketones from the breakdown of proteins in the body. Switch to unscented hygiene products and avoid perfumes.
There are a lot of passive traps that use rotting fruit for bait and you can use those to reduce fly population in an area.
However, most effective area defense for a home and even a vehicle would be the type of fly bait sold to protect cow barns. You have to buy it from agricultural supply. You usually use it around, but not inside of house but because it has an attractive aroma for flies. That's unnoticeable in a barn but not the living room. It's harmless to mammals unless consumed in large quantities and it has no transfer risk. (It's designed to be used around valuable and/or food animals.) I found this highly effective when we had an outbreak of flies here a few years back.
DEET won't help against houseflies because it jams the CO2 receptors for insects like mosquitoes that hunt mammals. Houseflies do not have those receptors.
I would note that in a suburban area, you really shouldn't have large numbers of flies. The two most common breeding areas are unsealed trash cans and the feces of large dogs. Check with your neighbors to make sure their trash is sealed and dog feces are picked up. All this should be done anyway as basic sanitation.
There is a folk preventative in the Southern and Southwestern US that says that plastic bags of water, suspended in the air, seems to drive off flies. It plausible that the in the spectra that flies see in, weighted to the ultraviolet, the refractive qualities of the water and plastic combined might cause a dazzling effect which the flies will interpret as predator motion. It seems to work anecdotally but I don't think anyone has actually bothered to research it scientifically.
However, it might be the only means of fending off flies in a vehicle. The enclosed space precludes pesticides of any kind so some kind of passive defense is required. You could try mounting a plastic water filled bottle to the dash and see if that helps. Bottle must be plastic because class is opaque to UV, so the theoretical dazzling effect won't happen with glass. I would suggest putting some aluminum foil underneath the bottle to prevent an accidental lens effect from concentrating sunlight to a dangerous degree on the dash underneath.
Phobias are a very real and difficult problem to deal with. I wish you luck.
