God! I can completely relate. Actually, just before I signed on here and saw your response I was reading a book called "Co-dependency For Dummies" and it's incredible. It's a great book with a 12 step program to help the reader recover.
Like you mentioned, codependency seems to share a lot with the assertiveness aspect. I think that someone can have both issues, and probably often do because they're both so intimately linked. I'm also attracted to Narcissistic people because I love to give. They love to take. It's that simple.
But like you said... it would be nice to just be able to give to someone like my heart wants to, and get respect back. Instead of mistreatment.