Dismissive attachment style is different from avoidant attachment style, which is different from Avoidant Personality Disorder. I doubt they are used interchangeably in scholarly papers. (If they are, that's a good sign whoever wrote it doesn't know what they're talking about.) That's a big problem when you search symptoms online - what gets returned isn't specific and it jumbles everything up, so it's really easy to confuse what you're reading. So for dismissive attachment perhaps self disclosure would be helpful, but for AvPD people, it could cause as many blockages as it opens up, and I think you agree with that.

I don't know what the aim of your therapy is, but it might be worth it to take some time in your sessions to work on the process of therapy itself. If you are holding yourself back because you're afraid of creating disappointment or being judged, then you'll make excruciatingly slow progress or even none at all. Your therapist is supposed to help you make progress, and if he can't adapt to make that possible (by reading your journal for instance) then you won't be able to break through your wall, and it might be that you're not as good a match with your therapist as you need and deserve and it's time to find a better one.