Is one who curses the religion, or the Lord, or the Messenger, may Allah bless him and grant him peace, in his heart without uttering it, then seeks Allah's forgiveness, considered a disbeliever, or is that considered among the pardoned self-talk?

You are not held accountable for these whispers; rather, they are from the plots of Satan. The remedy for them is to disregard them and seek refuge in Allah from him. Your dislike of these whispers is a sign of goodness and clear faith, because feeling them to be grave and fearing to utter them indicates the perfection of faith. Strive to get rid of them by frequent remembrance of Allah (dhikr) and performing ruqyah according to the Sharia. And beware of abandoning good deeds in response to them.