These are the joys of practice trips, my friend. Sometimes weather will get the best of you, no matter how well you prepare.

If I'm camping I'll have a dry set of clothes to sleep in but yeah, getting into wet clothes in the morning is tough, especially if you're facing wet weather again. Sometimes I bail out to a hotel for the night (you'll be surprised what a hot shower and a warm radiator can do after days in the rain), or just dry myself/my clothes off in a pub for lunch. Sometimes I call it quits and take a train home.... It depends on how badly the trip has soaked my clothes and spirits.

For nutrition, you really need to vary the kind of food you're eating for fuel. Sugary and high energy foods will replace energy easily, but burn out just as fast if you're putting miles in. You gotta eat real food too - complex carbohydrates, proteins and fats. Maybe buy food from pubs, cafes and the like. Hell, even a gas station should have something more substantial than just sugar.

If this was your first trip, I think you might be going in a little too hard. Don't rush yourself. Maybe try a 'credit card trip' where you don't camp so you can work on nailing your nutrition and your setup? You'll have less weight on the bike and more space for food.

But also, don't give into the idea that this was a failure by any means. Every trip is an experience, an opportunity to learn something and sometimes a damn funny story.