Maybe someone did not want to slow it down and grow a relationship at a comfortable pace. Some people don't even think about relationships; they are just looking for a flash in the pan. To me that person going away is a blessing in disguise. They took themselves out of it but left you without an open wound, just an inner ache.

Some of your questions would be good starters for a therapy session.
Here are some Psych Central articles, blogs and posts about anxiety and relationships.
Psych Central - Search results for How anxiety affects relationships