Even when gene tells Finny the truth, that he cause Finny to fall, Finny is unable to believe it and denies it. Why does he react this way? Why do you think you cannot believe the truth even when he hears it ?

Finny does not want his carefully constructed world destroyed. Gene was his friend and they had a rather creepy codependent relationship together. Finny does not want to believe Gene caused his fall because this would destroy whatever illusions Finny has built up about the two of them.