If I resist myself so as not to repeat the prayer, does my prayer become void or disliked, knowing that I am afflicted with doubt about the validity of the opening takbir, and when I resist this doubt, I feel as if I am talking to myself, which may lead to a loss of focus in the prayer?

The opening Takbir (Takbirat al-Ihram) is a pillar of prayer, and what the questioner is experiencing regarding doubt about it and repeating it is from the whispers of Satan. The way to get rid of these whispers is to ignore them and not give in to them. So, when you say the Takbir to begin prayer, do not repeat the Takbir, and continue with your prayer, and do not interrupt it thereafter due to doubt or inner thoughts.