What is the way to achieve sincerity of intention for Allah in good deeds, given the occasional feeling of a lack of sincerity, and is it obligatory to avoid good deeds in this situation?

We advise you to continue performing good deeds while striving against your self and correcting your intention, and to beware of its corrupting influences and the devil's machinations, such as abandoning good deeds for fear of falling into ostentation (riya').