Is it true that sins committed in solitude completely invalidate a person's good deeds, and how can one achieve salvation after repeatedly committing a sin and feeling intense remorse, especially since one's own limbs will testify on the Day of Judgment?

A person's deeds are not entirely nullified due to sins committed in private, except for those who fit the description in the Prophetic tradition: "I certainly know peoples from my Ummah who will come on the Day of Resurrection with good deeds like the white mountains of Tihamah, but Allah, the Almighty and Majestic, will make them scattered dust." These are those who, when alone with Allah's prohibitions, violate them. This does not apply to a Muslim who falls into forbidden acts due to overwhelming desire but then regrets it and tries to desist from them. Rather, it applies to the people of ostentation and hypocrisy.