There is no discrepancy. "But no man can tame the tongue... with it we curse men" contains a hyperbole.
Did the Apostoles or James curse man or couldn't they tame their tongues? Were their tongues a restless evil? I don't think so. James describes the situation that happens among Christians, but "these things ought not to be so" (3:10). He uses the word "we" not because he is a regular participant of the sin, but because he describes it as his own pain, and he stands with the brothers who struggle with the sin, avoiding the accusing word "you". But here again, he gives the hope that the tongue could be tamed (3:2):
If anyone does not stumble in word, he is a perfect man, able also to bridle the whole body
In 3:11-18, James explains again that the tongue should and could be tamed.
Of course, taming the tongue is not possible without God's grace, but the emphasis here is on human effort.
James 3:8 contains hyperbole and shouldn't be used literally without context.
Consider another hyperbole example (John 3:31-34):
He who comes from heaven is above all. And what He has seen and heard, that He testifies; and no one receives His testimony... he who has received His testimony has certified that God is true
Is there any contradiction here? Did really no one accept Jesus' testimony? Andrew? Nathanael? John the Baptist? "His disciples believed in Him" (John 2:11). They did actually accept his testimony. Still, his teaching was so high for humans, that even his closest disciples had problems understanding it, and Jesus felt lonely and unaccepted.