Why does Gene reverse his opinion about Finny, and why does this "menace" his understanding?

For some reason, coming to the realization that Finny wasn't trying to sabotage his grades does the exact opposite of what we'd expect and makes Gene even more jealous than he was previously. Gene's realization that he was wrong about Finny makes him feel even worse about himself; he becomes angrier and he experiences a higher degree of jealousy. His loss of his own sense of self made him everything he believed he hated in Finny, everything he believed Finny to be...... and this was a great menace to his own understanding.