HH Romapada Swami: Sincerity is what enables us to embrace the initial descending mercy we have received. That same sincerity is what Krishna accepts from those who have received some portion of His mercy but may continue to struggle with worldly thoughts, attractions, etc. even though continuing to strive for His shelter.
Chanting the holy name with undivided attention can be elusive to the degree that one may feel helpless and hopeless in one's efforts to chant without offense.
Especially Srimad Bhagavatam 11.19.9 can provide an invigorating source of hope and light for a sincere chanter!
"My dear Lord, for one who is being tormented on this terrible path of birth and death and is constantly overwhelmed by the threefold miseries, I do not see any possible shelter other than Your two lotus feet, which are just like a refreshing umbrella that pour down showers of delicious nectar."
"Having awakened faith in the narrations of My glories, being disgusted with all material activities, knowing that all sense gratification leads to misery, but still being unable to renounce all sense enjoyment, My devotee should remain happy and worship Me with great faith and conviction. Even though he is sometimes engaged in sense enjoyment, My devotee knows that all sense gratification leads to a miserable result, and he sincerely repents such activities."