you have to ask yourself if you really like this girl or you're just in it for the the sex. IMO she doesn't have any of those "special" feelings for you you she'd confide in you her problems and think that as her boyfriend this is what you should have to listen too. It sounds to me very much that you to are together because it's convineint not because it's what either one of you really wants.
Thought this situation sounds grim, I'd look on the bright side of things (Minus the painful sex). She's obviously still willing to be physical with you from time to time so you should take that as a positive. Maybe you could both be f*ck buddies or something of that nature. Frankly you're already on that track anyway so why beat aroudn the bush? Secondly, if she's not telling you her problems, thank your lucky friggin stars! I know tons of guys gettin NO sex hoping for it and having to listen to some whinny b*tches mindless dribble for hours on end. They tell me they want to put a drill bit to their head. Lastly, If you don't see this girl that much during the day, that means you have time to explore other options. This is the key factor in this relationship, since it apparently isn't going anywhere anytime soon, you should go out and find some one who may be worth your time or prehaps even a new friend who you could spend some quality enjoyable time with.
Either way things won't get better on their own and withou significant changes to your daily routine, or lack their of, this ones pretty much done. I say call in the salvage team see if you can find some sexual value from this lost relationship and call it a day. Or if you REALLY want to put some effort into it and make something out of nothing, you have to have a serious talk with her about your relationship and where you guys are then where you would like to be. Mind you this will lead you down the road of relationshipville and all the joys and pains that come with it so make your decision wisely.