If we pretend to smile, the reaction against other person deep inside our mind will make us produce a stern smile and a smile that is controlled which we called "force smile" which can immediately be identified as an unsincere one.
The method to overcome this feeling is by going through the source of our problem changing the negative image of our ourselves changing the bad impression of ourselves and finally by telling ourselves that we are not that vicious and that ee still have some virtues then smile at yourself.