You are not alone. I also hide my past history that involved substances during a period of time. Although I no longer have this problem, my experiences have caused issues that still have an affect on me.

As far as therapy goes, unless I'm going through a major issue, I am not really sure what to talk about. I feel that way today, actually, I have my appointment in a few hours. Notebooks have helped guide me about what I should talk about, although I haven't used this method in a while since it's still hard sometimes.

Writing things down has especially helped with my pdoc, since I can be quite disorganized and forget to share certain things that are important and leave feeling that I should have brought something in particular up. Maybe this will help you. I also agree with the above, you can ask the therapist if there is another form of therapy that may be more effective. Good luck with this.