Sincerity is indeed a difficult quality to cultivate, but it is a key to all the rest.
As a first step it helps to cultivate mental honesty, that is to say, to see things as they are without gloss or justifications. Then slowly one lays threadbare every strand of one’s nature to see the real motive behind all we think and feel and speak and do. One sees the forces that move us, often concealed behind neat acceptable facades. As we thus strip ourselves bare before the Divine and invoke Her Grace to remould us the way She would want us to be, then gradually things begin to become easy.
It takes great courage and determination as well as patience and perseverance to cultivate this indispensable quality and one can never have enough of it.