If she is saying that she is done then you need to move on, treat it just like any other break up. Don't become obsessive, don't get nasty, don't let yourself wallow for too long. Etc.

People with bipolar disorder are still people. We still fall in and out of love. We can still date someone for X months but it's just not meant to be. Sometimes we end relationships for reasons that are perfectly valid enough. Sometimes we fall for someone and it has absolutely nothing to do with our mental illness.

By blaming all of her feelings and her decision on her mental illness, you ARE treating her like a second class citizen. You are patronizing her, invalidating her and ultimately focused on what YOU want and not what she is expressing.

Yes, she clearly has some issues, especially if she is eating 400 calories per day. But arguing with her about her feelings is just. Just no.

Also, be aware that there could be a personality disorder at play such as BPD and that this could be a manipulation issue. Maybe she wants to watch you mentally sweat and struggle to try to 'save' her. Maybe that's why she told she's been eating 400 calories a day. But even if this were true, people with BPD don't manipulate to be "evil". They manipulate because it's one of the only coping methods they know as far as how to not feel vulnerable and try to get their emotional needs met. So even if that were the case, her emotions are still real, the way she feels is real, the thoughts she is having are real.

She does not want to date you right now, possibly never again.

You must respect this, for your own mental and emotional health. Treat it just like any other break up. If you would still want to be friends or leave the door open, let her know that if she wants to reach out sometime in the future, you will respond, but that you're going to take a little time for yourself to heal and move on. Then wish her all the best and do it.