You're talking about the heat and humidity killing you... Not to overstate the obvious, but can you ride earlier? Until you get a little stronger, ride in the cooler morning air.

I'd say most of all, keep at it! Those hard hills will get easier the more you ride. Make sure you're downshifting. If it's too hard, do an easier gear. I always find spinning at a high cadence (90-100, I have a cadence computer) on an easy gear makes life easier.

How long of rides are you going for? If they're over an hour, you'd probably do well to use sports drinks, gels, or snacks to keep your energy stores filled to give you the strength you need to get through it.