There is, sadly, only one way to treat agoraphobia - and all anxiety disorders for that matter. You have to go and do that which causes you so much anxiety. Every time you fail to do so, that validates your fear. The short reason is, part of your brain works to keep you alive, no matter what. But it can't observe the world. It responds to your feelings of fear, and it will not take chances. So when you give in to the fear, this part of your brain thinks you really are in danger, and responds in a way to prepare you for that danger.
Since that part of your brain only can interpret the world based on your active reactions, you absolutely must not give in to fear impulses. This is easier said than done, obviously. But its the only way.