Ahsan Allahu ilaykum ustadh, what is an advice to the person who believes they will never be able to become sincere due to their spiritual diseases? And if they feel they will never be able to be righteous because they always fall back into their old bad situation even after striving very hard?

Ustadh AbdulAziz al-Haqqan

This is just the Shaitan talking, concur your shaitan and you will be the best you can be in sha'a Allah.