How to Handle Drug-Addicted Mother and Her Recovery?

My mother has been a drug and alcohol user for at least 20 years that I know of. She has recently been ordered to a 28-day program, one year of structured living in a halfway house and counseling as well as some monetary fines for an alcohol-related accident she was involved in. She has not spoken to me in about 2 years because she has been so involved in her own world and has just recently written me a letter disclosing all that I’ve written so far and has asked me to tell her how she has hurt me in my life. I don’t even know where to start and I need some help in how to deal with her addictions, her choices and all that, how to answer this letter, how to ‘cope’ with all this myself and if, when and how I should participate in the rebuilding (or building) of our relationship.