Sexual intimacy is meant to unite a married couple and help them express their love for one another; it is also meant to be life giving, open to the gift of a child and to a greater sharing of their love with the world. In Catholic understanding, masturbation cannot fulfill these twin purposes of sexual activity. Because it is a solitary activity, it is necessarily inwardly focused and cannot lead to greater union with another. And, obviously, it cannot be open to the gift of life. While moral theology today recognizes that psycho-sexual development takes place over a period of time, and that immaturity can mitigate what is usually a serious sin, Catholic teaching maintains that masturbation is an improper use of human sexuality.
This is where your choice of words is telling. By describing your situation as “addiction,” you suggest that you have gone beyond the exploratory stage that is a normal part of adolescent sexual development. Your question suggests that you do feel conscious of serious sin. You may be experiencing precisely the consequences that the Church warns can come from a distorted pattern of sexual behavior: an impeded ability to form healthy affective relationships and negative repercussions in all areas of your life. If you really are dealing with a sexual addiction, seek out professional psychological help. A competent counselor can help you work through the issues that underlie all patterns of addiction, so that you can truly resolve to amend your life in the sacrament of reconciliation.