I have problems with this too. I agree with progaspie - please try not to be angry with or hard on yourself. Do you think that maybe in some situations, such as in meetings with your psychiatrist, you could maybe try writing down some things, e.g. what might come up in the session, and possible answers to questions? I know it's difficult to predict, but I know that when I see a psychiatrist it's usually 'how are you feeling?' 'how have you been?' 'any side-effects from the medication?' 'is it working?' 'is there anything you wanted to talk about?' etc., so I've been trying to write down in advance what I should say in response and anything I particularly need/want to happen, and anything I definitely don't want. Maybe your experience of psychiatric professionals is different, but sometimes I think that the way some of them speak and ask questions can be quite leading and make it harder to express what you need.
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Blackbird singing in the dead of night, take these broken wings and learn to fly...