I didn't have time to read all the replys so this may have been covered. I use to live in Quartz Hill (Palmdale) and in Bakersfield for a total of 15 years so I know what your talking about. 15 miles is not that far even in 100+ degree weather. So the question is how to stay on the bike for that long and longer without having to stop for 30 minutes.
First thing you need to do is prehydrate before you go, drink about 20 to 24 ounces of Gatorade full strength about an hour before you leave, then pee just before leaving so you don't have to on the road. If your averaging 15mph (which is feasible since it's mostly flat on the desert floor), then you need to consume about 5 ounces of Gatorade diluted 50/50 every 10 minutes which is 30 ounces an hour; thus 2 24 ounce water bottles are fine. Also you need to get your core body temperture down, so you will need to buy a couple of Polar bottles and fill one 3/4ths with ice then pour 75% strength Gatorade into the bottle. The other bottle you fill full of ice and pour in 100% strength Gatorade. On longer rides you will need a CamelBack (or something similar) and you fill this with about 1/2 ice the rest with Gatorade, then take along one or two of your bottles that you froze 50% strength Gatorade overnight solid in the bottle(s). When you finish riding drink another 20-24 ounces of Gatorade 100% strength over a period of an hour as soon as you get home.
Next you need to keep your outside body as cool as possible. Wear white 100% polyester jersey's which Walmart sells cheap ($9 to $12) called DriStar and they work just as good as the expensive jersey's (I own both kinds and can't tell the difference); the white does reflect some of the heat away. Also buy a loose jersey not one of those form fitting kind, the loose jersey allows the air to bellow into the jersey causing a cooling effect. My experence found the form fitting jerseys to be hotter then the loose ones. Also try to find a loose fitting 100% polyester short (Walmart does not carry), I found MTB shorts to work very well in this manner, but I also use standard MTB looking shorts that don't have the crotch padding (Walmart does have these) that work better in the cooling department as long as the ride doesn't go over 40 miles then your going to wish you had the padding!
Your helmet needs to be very well ventilated, and use a sweat band that will go around your head between the helmet to prevent sweat from dropping into your eyes.
Now here is a interesting problem, if you use sunblock to prevent the intense sun from damaging your skin your doing yourself a favor in preventing the possiblity of skin cancer later; but sunblock actually makes you hotter! so the choice is yours...in that case I would rather be hotter then worrying about skin cancer.
Also wear full wrap sunglasses to prevent your eyes from drying out and or getting sand in them.
2pm is a bad time to riding home out there, not so much because of the heat but more because of the wind that kicks up between 2 to 4 pm almost everyday. That wind I hated due to the blowing sand and tumbleweeds so I avoided riding during those hours, but I know that may not be possible for you. You may want to bring along a cloth hankerchef so you tie it around your mouth and nose in case it gets real bad. I was there in 1991 when the wind blew so hard it looked like we were living on Mars! and even after the wind stopped around 5pm it took till 7 for the sky to clear up!!
Also (if you haven't already done this), that area is littered with thistles and goatheads all of which torment tires like crazy, I used Specialize Armadillos and never got a flat. This is important because it's a pain to fix a flat when the sun is beating down on you and it's over 100 degrees.
But your tiredness is probably due to the lack of sodium or Potassium and Gatorade provides that for you. You can also supplement that with Potassium supplements from Stim-O-Stam if you find yourself having muscle cramps. The heat will strip your body of your electrolytes and you need to replace that, water will not cut it.