It sounds to me that's what your therapist was referring to by saying "You have to be more aware of your amnesia of your amnesia".It sounds like you have amnesia but you are unaware that you do(therefore amnesia of your amnesia).

I think your question should have been how do you learn how to become aware of it,what can you do to help yourself with that,etc.

If your dissociation is so severe that you are unable to complete assignments(or to even remember that you are given assignments)recording sessions might actually be a good idea to help you.Or maybe ask your therapist to put it in writing for you or email or text you with reminders or something.

It sounds to me like your therapist(s) is/are trying to find ways to help you but of course it's up to you whether you decide to find a different one or not.I know many things my therapist said didn't make sense to me and frustrated and irritated me at times but I had to trust that he knew what he was doing.

When you ask "How do you get people to understand that you don't do it on purpose, it isn't a convenient excuse, and that I have an actual disorder",were you talking about your therapist or people in general?Is it your T that is scoffing and invalidating you or other people?