There are very few ways in which you can actually fight fears. In Harry Potters world most fears take on physical form. A Dementor or a spider can be fought. What if your fear is not tangible? How do you fight a fear of heights, for instance?
It is my opinion that most fears cant be fought. They can be tempered, faced, and lived with. I for one have a phobia of snakes. I have handled it by learning about them, and watching them at the zoo. My fear has never gone away. I can tolerate it and deal with it, but there is always that little twinge of panic in the back of my mind telling me to run. Mastering this impulse is the key to dealing with fears.
To understand how this works, lets take everybodys favorite case study: Harry. His fear is a Dementor. I think he is afraid of this because he didnt know how to fight it. Harrys real fear -- again, this is in my opinion -- is the unknown. What he cant see and what he cant affect. It was the helplessness that got to Harry more than the actual Dementor.
Once he learned the Patronus spell he was able to handle Dementors and they really get demoted from a fear to a hindrance. Think of the last Tri-Wizard task. He comes across a Dementor and goes into pure reaction. Only then does he realize that its a boggart, not a Dementor. Thats Harrys mechanism for dealing with fears. React. React. React.
Harrys real test will come when he has to deal with the unknown. Voldemort is the master of moving unseen and working behind the veil (no pun intended). How will Harry deal with this? His reactions have been very formulaic. Learn about it, learn to beat it, and then beat it. What happens when he is faced with a fear so unknown, nobody knows how to fight it? He has friends that are good for that. Ron has good instinct, and Hermione is good for anything in a book. That pretty much covers his bases.
What about when it comes down to it and he is alone? Like in the graveyard in GOF, he was saved by a coincidence. How many of those could he have left? Each time he has faced Voldemort (the real one, not a memory mind you) he was saved by an unknown force. In the first book, Voldemort could not even touch him. Now that protection is gone. Their wands are no good against each other. What has Harry really got to throw against Voldemort any more? Nothing.
A strong since of love and courage will only protect him so far. After Order of the Phoenix I think Harry is starting to realize this. With Half-Blood Prince coming out you have to wonder how Harry is going to react to that. How scarred will he be during his next inevitable confrontation with Voldemort? If you ask me, the next time Harry sees a boggart it will take the form of a Voldemort that has Harry out done in everyway, or a Voldemort with a new weapon. Remember how much that idea bothered him in Order of the Phoenix? I have a feeling that idea will come back in the next book.
Ah, but Harry has one weapon he has not thought of yet. He has a talent -- granted an uncontrolled one -- for throwing out magic without a wand. He has done it twice so far in the books. Think about it for a second. That could be the key to bringing down Voldemort. That anger that seems to be permeating Harry now.
With Sirius gone it is only a matter of time before something pushes him over the edge. What will that horrible event be? That should be Harrys biggest fear. The day when he will actually want to murder Voldemort. Something will have to cause that change in Harry, I dont think it will be a good thing for Harry or his friends.