How can one purify his intention for the sake of Allah Almighty, despite feeling it is impure, even with repeated attempts?

Sincerity of intention to Allah means that the servant, through his charity, desires reward from Allah, not from people. Sincerity can be achieved by seeking Allah's help and striving against oneself to repel the whispers of ostentation and seeking renown. One should not abandon good deeds due to not feeling sincere.