Don't sweat it. Everyone gets dropped sometime. First time I went on the South River Ride, I got dropped literally in the first minute. Next time, I tried to hang on a little longer, a little longer after that, a little longer after that. Most riders I know have stories like that. Next time give yourself a better warmup and maybe take a bar or banana to eat in the parking lot or on the easy parts. Hang on as long as you can. When you're cooked, sit up, have a drink, and ride tempo home.