Isn't this why most instructors (that I've had at least) insist upon lots of repetition? I.e you keep going over the same move many, many times. Not to bore you, but to train the muscles to just do it.
In a real fight, there is no time to think. When a punch is coming towards you, you don't have time to decide what is the best block and counter. You only have time to DO. What you do may not be what you've been taught, but it will be instant and autonomous, and if you've trained the right reflexes and the right muscles in class, then some of it might show a hint of its presence when the need arises, but I wouldn't count on it.
Its years since I've been in a genuinely hostile situation, but I remember when I was, everything happens so fast there simply is no tensing up. Half the time you don't get any say in what happens, your flesh just does what it needs to do to protect itself and the terrified mind is just kind of along for the ride.