And so said all the disciples. Peter was trusting in his own ability and resolve. You are weak in the flesh. The spirit indeed is willing, but the flesh is weak. Then he went out and wept bitterly.
Bitter weeping is normal for someone whose pride has just been broken.
Peter was suffering the pain of failure. He realized he was wrong in what he did, and in what he thought about himself. You must trust in God and His Word. Guard against slipping into trusting your knowledge and ability. To prevent falling, maintaining the habit of continual prayer is necessary.