What should I do when I feel a lack of sincerity in my work, even when I am alone?

A person's feeling of shortcoming and lack of sincerity is a good thing that calls for perfecting sincerity and improving intention. However, it should not reach the point of obsessive doubt (waswasah). Moderation is necessary, along with having a good opinion of Allah and relying on Him to grant you sincerity and accept your deeds. Do not let Satan prevent you from doing good deeds under the pretext of lacking sincerity. Rather, do good deeds, seek Allah's help, be sincere to Him as much as you can, and ask Him for acceptance and success.