I am sorry. The only thing that I can suggest is posting on

http://forums.psychcentral.com/anxiety-panic-phobias/

asking for help. On that forum, there are people who confine themselves to their homes and apartments. But at least they WANT out! They may not get out, but they want to, and when they do get out for a quick stroll around the block or to buy milk from their local grocery store, they post their victories and congratulate one another on their achievements (which may seem small but are not small). So they have a desire to get out that your gf seems to lack. But ask them - maybe some of them, at some point, did not have any desire to go outside of their little cells. I think you might get more help from them because the issue is not narrowly sexual but broadly "somebody has lost any passion for life".