I posted this little bit in a response to someone else long ago, and I'm copying it here for relevance:
When you walk into a relationship with someone with a mental illness, the best you can hope for is to walk into it with your eyes open and you have to be prepared to make sacrifices. One lesson I've learned is that we are never meant to sacrifice anything that is necessary to our being, but instead we must sacrifice what is superfluous to our needs.
It's their own private war, really, and as much as you want to fix it or help or make things better, you can't. It's very frustrating, because you think if they love you, it should make it better, just love should make it better, your presence should make it better. But it can't. Not with mental illness. It's so difficult to love someone who dissociates, because they're always going away from you; even if they stay with you physically, they retreat into their own mental patterns which have nothing to do with you or your life. You feel shut out, and you can't get in. But...this is where the test starts, and this is where the real love begins. Because if you really love someone who is sick, you have to stay there, open heart and open mind, ego issues be damned. It is an illness, and, as with a physical illness, you have to focus on what's best for them. Usually, the thing that works best is letting them know in no uncertain terms that you're there and you're not going away--that their darkness doesn't scare you. That your love doesn't depend on whether or not they're having a good day or a bad day (a hard lesson for me). Physical contact is tremendously important--you can't be shy, or expect them to take the lead most of the time. In fact, you can't have any expectations at all. When you love someone who is prone to psychotic breaks or bouts of depression, none of the usual relationship patterns fit. You have to throw your own ego out the window and you have to find your own way through it as a couple. You have to remember that they tear themselves apart on a regular basis, and while we can't put them together again (no matter how desperately we want to), we can in fact guide them towards putting their own pieces back. Here's the clincher...it's very easy to get lost here, feel as if you're giving too much and getting not enough in return, it's very easy to feel hurt and neglected. But if you really love someone like this, you have to grow up fast, swallow your pride, and just know somewhere deep down that this is the right thing for both of you. That you are the right thing for both of you. And that will get you through.