Yes Jal, outside is very intimidating. I live by myself in a very cluttered house and my brother stays regularly to take a break from the family. (I have 6 siblings including him). As intimidating as it is, I can brave it for almost 5 minutes. I need to be in a secure building/area before I start to calm down. I have a car, so I practically walk for about 30 seconds to get to it. It's something that took me years to accomplish but it's been done now and I do it almost every day.
It is also why I probably no-life on CoD for most of the time my brother stays with me.
What happens, in short, is you are a regular person, but when you open the door, your mind starts thinking of strange things that could happen that probably have a 0.0001% chance of happening. You just stand there and think, I can't go out there, I don't have the courage. I put it as 'Fear of Heights' since that's the closest thing there is to Agoraphobia that I can relate too. It's like, for you, standing on top of a really tall building and looking down onto the city. There's a building next to the one you're on that is a little bit shorter and the gap between it is small, less then 2 feet wide, and your friends have jumped over the gap, but because you're scared of heights, you're always thinking "Im going to fall and die" and you have no self courage. That's how I feel about outside. I stand at my door and say "Im going to get hit by a car again and die" or "A plane is going to crash and i'll die" It's not likely to happen if you use caution but it's the courage you need to build up inside yourself to be able to push yourself to say, "Hey, that's only a small gap, I can jump that" or "Hey, if I stay off the road or away from it, I wont be hit by a car".
That's probably the best way I can put it for now Jal. I hope it helped. :]