A teacher who doubted you handed you a free rival. Use them well.

Doubt from someone with authority stings because you did not choose them and cannot easily argue. It can also be converted, in the sense that a specific person who expects little is a clearer target than a vague wish to do well. The energy is useful right up to the point where it becomes the only reason you are working.

Use it this term, then check whether you would still want the result if that person vanished. If yes, keep going for your own reasons. And if they turn out to be right about a gap, take the information and leave the insult.