An idle mind is the devil's workshop. Surfacing of unfavorable thoughts or fears is an indication that the mind is not appropriately engaged in positive devotional cultivation.
The first and fundamental step in bhakti is 'Sravanam'. Other processes of devotional service, such as chanting, praying, worshiping etc should be *founded* on the principle of sound hearing. We must hear repeatedly from Pure Devotees who are themselves free from all offenses. We must chant and serve Krsna with their guidance. Please, therefore, take firm shelter in systematically studying Prabhupada's books and/or hearing his lectures. Negligent or sentimental practices of devotion, without help of deep understanding of Krishna based on scriptures, will leave our devotion weak, and prone to the attack of so many misgivings.
We have forgotten Krishna for so long, and have associated with heavily materialistic and atheistic conceptions. Offenses, or the tendency to commit offenses, is due to the lingering material conceptions and ignorance only. They cannot be overcome simply by fear or forceful efforts to banish offenses. Instead, the effort must go into cleansing the material contamination and misgivings in the mind. This is made possible by constant hearing from suddha-vaishnavas about the transcendental characteristics of Krishna.
mayy asakta-manah partha
yogam yunjan mad-asrayah
asamsayam samagram mam
yatha jnasyasi tac chrnu.
"Now hear, O son of Prtha, how by practicing yoga in full consciousness of Me, with mind attached to Me, you can know Me in full, free from doubt." (BG 7.1).
When knowledge of Krishna is properly awakened, free from all doubts and misgivings, one will automatically become free from offenses, fears, rebellious tendencies and all their corollories.
Krishna is extremely merciful -- He overlooks our childish mistakes, especially of those trying to come back to Him. Despite their ignorant mistakes or offenses, such devotees are as much fortunate as a millionaire's child, in comparison to a lost and orphaned child who has no one to point out his mistakes!
- HH Romapada Swami