I struggled with this as well, having been raised as a christian.
My take and undersatnding on this passage works for me now. "Honoring" my parents doesn't mean following and participating in their sickness. It means that I allow them to live the lives they're living, and put a boundary to protect myself from their sickness. It means that I "forgive them for they know not what they do", but it does not mean I allow them to trample over my human dignity any longer.
Our parents are sick people, and their recovery is their responsibility. If they're not able to take responsibility for their lives, that is their choice.
Our parents have the power to choose their own lives, and our acknowledgement (and letting go) of this is "honor". We "honor" their dignity as human beings, even when they cannot "honor" our dignity as human beings.
It's a heroes journey, and you are the hero.
Loving Kindness Meditation will dramatically improve your spirits; give it a try for just 3 days: http://www.youtube.com/watch?v=sz7cpV7ERsM