Best fly spray/homemade fly sprays for really bug sensitive horse?

at my trainers barn i have access to all the horses whenever i want, so i have many horses to ride as well as my own. the other day my trainer asked me to ride the new horse who is a 4 year old standardbred who she rescued from going for meat. i groomed her and put her tack on and fly sprayed her, and she was fine, but when we got outside the bugs were REALLY bad and she started flippin out and rearing while i was walkin her around before getting on. then she kept pulling away and started foaming from her mouth and chewing on the bit so hard i thought she was gonna chip a tooth or something lol, but i decided not to ride her because she then started sweating and was way too stressed and i didnt feel like taking a chance by getting on her. i took her in the barn and she was absolutely fine because the fan was on so there werent any bugs in there, so my question is: what would be a good fly spray for her? im open to store bought ones or home made ones, just something that might help her get not so stressed out next time :) thanx in advance