You need ability to work as an ATCO, but you need the right personality to harness that ability to get through the training and validate.
Trainees who know the limits of their abilities are the most likely to succeed as they will embrace criticism, learn from their mistakes and genuinely recognise when (and why) they have improved. Trainees who think they have no limits to their abilities will reach a point beyond which they cannot improve because they will not listen to what they are being told and will fail to learn from their own experiences.
Validating as an ATCO requires a lot of hard work, a great deal of critical self examination and an almost infinite capacity to learn from your experience and the experience of others. Without these traits all the ability in the world won’t help