Must a worshiper who feels a lack of psychological reassurance towards the Imam repeat his prayer?

One must think well of the Imam, for the presumption is that he is upright, especially if he is among the people of knowledge and apparent righteousness. This is because having a bad opinion of a believer is forbidden, in accordance with the Almighty's saying: "O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin." And the Messenger of Allah, peace and blessings be upon him, said: "The sanctity of a believer is greater in the sight of Allah than your (the Kaaba's) sanctity... and that one should not think of him except good."

This does not prevent one from seeking the best Imams to pray behind. However, if there is only one mosque available, do not abandon congregational prayer behind the Imam. Nevertheless, you may discreetly offer him advice if you are certain of your observations.