Don't discount your child's fears or attachments
Children can develop some fears during potty training, and they are as large to them as any fear adults could imagine having. Children may not understand the mechanics of the toilet and that large flushing sound in that small space can be frightening. If a child experiences even one slip off the toilet seat and bottom touches water, it can set them back to square one or even require a potty training haitus. Some children have a hard time dealing with watching their poop disappear down the drain, as if it were as much a part of them as an arm or a leg. Treat these fears with sensitivity. Discuss the fear without invalidating it or making your child feel shamed or as if his feelings are unimportant. Some children may need help expressing their fears, so offer them the vocabulary that seems appropriate. The same is true of attachments children may exhibit during this time. Diapers may represent a feeling of security in many ways. It is a time when parents are intimate with their children and are taking care of their needs, and letting go of that takes some children more time. This doesn't mean letting your child go back and forth between wearing diapers and training, but it does mean making sure he is ready to take that step of independence. If he seems to want to cling to diapers, suggest that it's potty time and tell him that afterward (whether he's used the potty or not) he can have a story on your lap or have some tickle-time. It may not be the diaper he's missing, but rather the closeness with you. It can be cold and lonely in the bathroom, after all.
Don't worry too much
Really. You've probably heard it or read it a million times by now, but it's true: It's highly unlikely that your child will go to college in a diaper. (Although I have heard some horror stories about people rushing fraternities.)
Try to take potty training in stride and work together with your child. Look at it like just one more learning opportunity, one more step of growth and independence in the life of a very special person in your life. If it helps put it in perspective, think of all the new and scary times you've had where you were equipped with the coping skills of an adolescent or adult -- a new job, a new relationship, learning a new language, giving a speech, taking a driver's test. Then imagine doing those same things without the matured coping ability you have today, with the motor skills of a small child, and a head so big you still can't get your arm up around it to touch the opposite ear. Treat your little guy or gal with the same patience and encouragement you'd want in the same situation, and don't forget to still have fun along the way.