'Bitter' in the Bible

Just consider and meditate on Him who endured from sinners such bitter hostility against Himself [consider it all in comparison with your trials], so that you will not grow weary and lose heart.

See to it that no one fails to obtain the grace of God and that no bitter root grows up and causes you trouble, or many of you will become defiled.

For you know that later on, when he wanted [to regain title to] his inheritance of the blessing, he was rejected, for he found no opportunity for repentance [there was no way to repair what he had done, no chance to recall the choice he had made], even though he sought for it with [bitter] tears.