It may not be that he doesn't understand what you're saying, but he doesn't agree. A therapist may not always agree with your assessments about yourself, particularly if they are about negative traits or certain things you think you can't do. Perhaps what he meant by "open book" is that you are honest about what's going on with you, and perhaps he thinks that you open up so well it must be relatively easy for you, at least compared to other people who have trouble talking at all.

I think it's a good thing when a therapist believes his client is more competent and she is also capable of doing things without such difficult that she falls apart. To hold a sense of a person at their best rather than collude in a belief that everything is so hard for you and you're not good at this or that or can't do this or that because of your diagnosis or problems is a kind of inspiration.

But it's is also not your therapist's job to agree with you, about yourself or about anything. And how would him "understanding" that it's too hard for you help you move forward with your issues? You might like the validation, but is he really stopping you from moving forward. Couldn't he, in fact, be encouraging you on?