Hi,
Your nagging habit will count against you in teaching a class of young kids.
Your constant worry will count againt your health.
Do the above two sound right and good?
To me, they don't sound totally wrong but they don't sound entirely right. 'To count against' is better used in a situation where you are more clearly going to be judged by someone in some way.
eg In the eyes of the jury, his fingerprints on the gun counted against him.
eg Tom's constant interest in other women counted against him when he asked Mary to marry him.
The opposite would be 'counted in his favour'.
Best wishes, Clive