Yes, they might lose heart or forget the purpose of the quest they're on but there will always be someone to remind them at crucial moments. Okay in life we might have people who will remind us what the point of it all is for us but that is not definite in the way it has to be in fiction. If Character A forgets why they have to get rid of an evil ring, Character B will ensure they're reminded of it in no uncertain fashion.
So who in your story acts as the guide to your main character? Does your main character lose sight of the point of it all and how are they helped to regroup and refocus?