I'm sure you understand this on a logical level (though perhaps not emotionally, which is the one that really matters), but the mechanics of executing a mark with a fineliner is no different than doing so with a pencil. Your arm moves in the same fashion, the tool makes contact with the page, and a mark is made. Of course, what interferes on the part of the fineliner is the psychological aspect - we know we're committing to a permanent stroke, we know that mark is going to be fully dark and unavoidable. It causes us to fear the process of mark making, and this is what causes us to second-guess ourselves.

We know that we can accomplish the task, but the specific nature of the tool is what hinders us. It is a matter of coming to terms with the fact that there is no real difference between what goes into making a mark with one tool or the other. That the difficulty involved, and the frustration is something that we bring to the table ourselves.

On another note, just as a formality, I did want to confirm that you are indeed doing warmups of 10-15 minutes, consisting of 2 to 3 exercises chosen from the lessons you've already completed? Those earlier exercises are the bedrock of these mark-making mechanics, and all students need to continue refining and sharpening those skills as they move forward, both to continue improving and to maintain what they've already developed.