Codependency refers to a pattern of problematic behaviors in a person who is in a significant relationship with someone who has either an addiction or other identifiable, fairly significant emotional problem. Co-dependent people are often so focused on trying to help and fix the other that they they lose track of their own needs, as well as their own areas that need growth and change.
Co-dependent people often can't tell what is their fault and what isn't, and what is their responsibility or someone else's for problems in the relationship. It involves a tendency to over-function for the other person. It is a felt sense of responsibility for the other in a manner than is too much, and ultimately, bad for them and for the other person. It involves a desire, sometimes a crusade, to fix, cure or change the other. It also involves a tendency to blame oneself for the addictions in another and to experience inapprioriate guilt for the probems in the addicted person.
In programs such as Al-Anon, the '3 Cs' are emphasized - you did not Cause the problem in the other; you cannot Cure them, and you cannot even Change them. What is important is that you do not organize your emotional life around what the other person is doing or not doing. It is really important to be in touch with your own feelings and to have healthy and appropriate outlets for their expression. Behind every well-intentioned co-dependent person is someone who is angry and resentful.
The best way to get rid of the anger and resentment is not to demand change in the other. Rather, anger and resentment go away when you do a better job of: 1) managing your emotional boundaries; 2) getting the support of others; 3) recognizing your needs and seeking healthy and appropriate means of having these needs attended to. For more assistance in this area, attend Al-anon or other comparable groups; in the Chicago area, also consider a professional consultation at http://heritageprofessional.com