This is about training, it is not about a rule

Aristotle's point here --- that choosing the middle ground is best --- is not intended as a moral rule. Rather, it is intended as a training principle. That is, it is not that you are 'good' if you choose the middle and 'bad' if you choose too much or too little. Aristotle is saying something quite different. He is saying that when you train your emotional skills, you need to aim (roughly) for the middle as you practice. If you always do too much in your training, or if you always do too little, you aren't habituating the middle ground. So this is about training, not a rule.

Also, Aristotle points out that the practice arena is alway the same, irrespective of whether you practice in the right way or the wrong way. For instance, if I were learning to play the piano, I would sit in a practice room and practice every day. However, I could still be practicing in the wrong ways there. So simply jumping into the practice arena and practicing is not enough to guarantee that I will develop my skills correctly. It takes the right kind of practice.

Generally, Aristotle believes that what makes you 'good' or 'bad' is not following a rule. You are 'good' or 'bad' if you more or less consistently do the right thing. You can slip up every once in a while, especially while you are training. But slip ups don't make you 'bad'. Excellent piano players can make mistakes once in a while. What makes you 'bad' is consistently doing the wrong thing. By Aristotle's reckoning, you are judged to be good or bad in accordance with how you live your whole life, not in accordance with how you enacted this or that particular thing.