Definitely make the best of it! Never worry on things that you can't control. Off topic, but before I got married, friends and family from both sides kept asking us " Aren't you nervous that it might rain on your wedding day?". It really became annoying after awhile, but I kept telling people that no, I wasn't worried, because worry or not the weather was going to do whatever it wanted, so why bother?
Same thing with Disney. Don't worry about rain, heat, crowds or anything else. It is what it is, you will make the best of whatever comes up, and have a great time!