I have a very similar problem of blocking. Weed also makes mine worse. However on stimulants I can say everything I want to say. Even coffee helps for a few hours. Meditation didn't help the blocking nor did learning not to fear the block. I stopped giving a fuck about the block a long time ago; no fear exists. I do not care what people think of me or my speech, or how people see me as a person. Like yours mine can be better/worse depending on the situation, such as when trying to tell a story or explaining something. But the worst seems to be direct questions where I can not substitute words. It is still present in all situations though. The only time it is not present is under the influence of stimulants, which could suggest it is anxiety based (subconsciously as I'm not consciously anxious) or an imbalance of dopamine (the stimulants I have used work on dopamine). Mine started quite late at the age of 18 after a stressful period of my life. My first thought would have been to remove stress would remove the block, but this hasn't helped.