You have a phobia? Well don't worry!
Firstly, talking with someone always helps. Since you're afraid of being alone, try to have someone, like a friend or family member go with you to places. Phobias are hard to overcome and talking to a counselor is the best way for guidance. You can also take steps, such as writing in a diary about your feelings!
Here's a 14 step guide to overcoming a phobia: it's not in one day tho
Google "Overcoming Phobias" too, as this all can help you and you can pick which one works for you!
It's a pretty common phobia. As far as why you don't know, it could of developed or it wasn't inflamed so much as it is now while you're a teen.