I've been told that 3/4's of addicts/alcoholics also suffer mental illnesses and that 3/4's of mental patients do/have self-medicated with drugs/alcohol.
All I know is that I rehabbed twice for drugs/alcohol with no good coming of it. The third time I was admitted dual diagnosis (Bipolar II) and that time it worked. Seven and a half years sober and still on the meds that keep me sane.
Good luck to you on your journey!