Sometimes the word "just" suggests that only a small difference would have had a major effect on the situation.
The train left five seconds ago. You just missed it.
I stretched my arm through the bars to pet the gorilla, but it was just out of my reach. Another inch closer and I'd have done it.
Sometimes the word "just" suggests that no matter how much effort one has put or could put into something, it is not going to be possible. Even a major difference would have no effect.
I stood on a ladder and jumped. I just couldn't reach that ledge. The prison wall was much too high. There was no way I was going to escape over the wall.
In your example about articulating ideas, the meaning is the latter. The words would not come at all. It was not that they were on the tip of your tongue, or that you could not find the perfect way to state the ideas; rather you found yourself wholly incapable of expressing them. The words simply would not come. The usage there implies utter frustration and futility, a great distance between oneself and one's goal.