OK. Might be a good idea to get a cuppa - this is a bit long!
Imagine that your mind is a ship, with a captain and a crew.
The captain is your logical, conscious, objective mind. The captain decides where we are sailing to, and why we are sailing in that direction and what we're going to do when we get there. In other words achieving our short, medium and long term goals. There is, of course, only one captain and s/he can only work on a limited number of thoughts at any one time.
The crew is our subconscious mind, they steer the ship and carry out all the day to day tasks we are not consciously aware of; all our automatic thoughts and actions. They are also the custodians of our emotions, our creativity and our imagination and (most crucially) our core beliefs, which is the way we view the world and our place in it.
Up to the age of around 9/10 years old there was no captain on board the ship because children don’t have the capacity to consider the validity of what they are told or to think about long term goals. So the crew lived in the moment to keep the ship afloat as best they could, trying to keep us safe and happy. At the same time they absorbed lots of information, ideas and opinions about who we are and how we fit in the world. Some of this information was correct, some of it wasn’t, however it was all taken on board and believed to be absolutely true, both then and now. We call this the map of reality or critical faculty.
When we mature and try to do things that the crew decided earlier in life were not a good strategy, our ship hit the rocks. It could be something simple such as accepting that spiders are not dangerous or it could be life changing, such as standing up to a bully or giving a confident presentation to a 100 colleagues.
With agoraphobia, and other phobias, something happened (which could have been so minor you can't now remember it, or could be seared into your memory) and a crew member decided that that was so scary that it was going to take on the responsibility of protecting you from ever being in that dangerous place again. That crew member is doing its job every day because it believes this is the best strategy for the most happiness.
I'll give you a silly example. When I was a child I spent a lot of my on a nearby farm, playing with the animals and generally making a nuisance of myself. One day the farmer jokingly threw a small worm and told me it had landed in my welly. I was hysterical then and, to this day, always shake out my boots...just in case there is a worm lurking in there!
So how do we persuade the crew to stop doing what they're doing? If you know what your particular trigger was then consider what happened, when it happened and what was the outcome. Look at it from every angle now, as an adult - not with a child's eyes. How likely is it to happen again? What steps can you - as a grown up - take to ensure your safety? Did anything truly bad happen or did your imagination concoct a series of events that could have, but didn't actually occur?
Now (and this is when it sounds a bit mad but if you've made it this far you might as well carry on!) try and talk to the crew member in charge of your phobia. Start by thanking it for having done a fantastic job but explain that the work it's doing now isn't making you happy, in fact it's actively stopping you from doing the things that would make you happy and you'd like it to stop now. Have this conversation out loud or in your head. What will generally happen is you'll get lots of thoughts popping into your mind as the crew member panics a bit and tries to remind you of all the scary memories. Let your captain (your conscious mind) counter these fears with facts and rational thoughts. Keep going until you can think about the phobia without emotion.
Phew. I need a cuppa now.