How is sincerity achieved, and are acts of worship performed during the stage of striving (mujahadah) accepted before reaching it?

To achieve sincerity, strive to perform a good deed secretly, unknown to anyone, as the Prophet, peace and blessings be upon him, said: "Whoever among you can have a hidden good deed, let him do it."

Know that a servant does not reach a degree of sincerity where he is safe from ostentation (riya'); rather, even the great ones may experience thoughts of ostentation.

As for the acceptance of deeds, its knowledge rests with Allah the Almighty. The servant should perfect his deeds and hope for acceptance from Allah.