100 degree in Texas here. I didn't read all the posts so sorry for the repeats.

You have to acclimate, make sure you are drinking something other than water and I have to up my intake. Pour water on your head (you are going to look like crap when you take off the helmet anyway). A Buff (look at REI for them) is great as a skull cap with water on it. Put ice in your sports bra if you stop for a refill, it is magical. Also I have to have something with sugar when I am done like a coke. I really love my Camelbak bottles (non-insulated) so freezing them helps. I also slow down, the heat is brutal and you can ride all day in it if you are smart.