Re (from corey, Post #26
"Right now I feel like I am somewhere between 'content' and 'happy' ..."
If that's the case, then you're fine to stay in the relationship for the time being. Keep "checking up on yourself" and if you find you feelings change to depressed, desperate, or what have you, then it's time to make some hard choices.
Make sure you're being honest with yourself, and with her. I personally don't see anything wrong with being a "nice guy;" I think you can be a nice guy without being a doormat. Just try to balance things out in your own life.
D should be told the truth eventually; he doesn't need to know everything about you, but he does need to know that you're there and that you're more than just a friend.