There will be times in a relationship where a girl will start to doubt things, she'll start having second thoughts, she'll start to wonder if staying together would be the right thing to do. That's when you have to reassure her that everything will be okay, that arguments are normal, that things won't always be perfect. Show her that she shouldn't give up on something just because things get hard, that as long as you don't stop trying, she shouldn't either. If she sees that you want to work things out, then she won't feel hopeless anymore.
p/s; HAHAHA! it happened to me all the time :D