From the few women i was with who I dated, i think that all relationships are bonkers and wacky, so your situation might not even be that unhealthy all things considered. As someone who has been sexually frustrated for a long time, i would almost say keep it going as long as you can. It is unethical but there are worse things both of you could be doing, you know. If you are an emotional wreck and crying and sick, that to me is more of a reason than anything else to break it off with her and start the healing process.
In your situation, it seems like the type of thing that will implode on it's own at some point, so i would just have fun with it as long as you can, and emotionally detach yourself from her, or the possibility of being with her. Then when it really is over, you can say you got as much out of the situation as you could have (not just strictly sex, everything)