Re (from evad
"So, what you're saying is either I break up with her (and break my heart) or it's possible down the road she'll break my heart. There's really no downside to stringing this out, as far as I can tell."
Well, the possible downside would be if there was another potential relationship out there somewhere that you could be investing your time (and energy, and emotions) on, and maybe that other relationship would be healthier (and less owie) than this one. On the other hand, if you like the relationship you have (overall), then there's no need to go through the trouble of looking for another relationship.
As for how to communicate with someone who has depression, you've pretty much already got the right idea in that it helps to just give the depressed person some time to process their feelings. Oh and one other helpful tip: Don't try to "blow sunshine up their ass."
In other words, don't tell them, "Oh honey, there's no reason for you to be depressed, just think of all the good things in your life." That just makes them feel like their feelings have been invalidated, and might even make them more depressed (and irate as well). Just acknowledge what they're feeling. "I can see you're going through an awfully rough patch right now. I'm sorry about that. I just want you to know that I'm here to help if there's any way I can." And then as we said, give them a chance to process their own feelings.
I speak from some experience as I've had my own struggles with depression (as well as other junk like bi-polar and whatnot). So this info should help, I hope.
As for communicating with a "white liar," the others have already covered those bases well. There's books you can get and that sounds like a good idea. But to a large extent, you'll just have to try to gain a zen acceptance of it, as much as you can.