When people betray us or turn against us, I know it's natural for us to feel bitter. And along with our bitterness often comes a host of other emotions: anger, resentment, a yearning for revenge, feelings of rejection or worthlessness, and so forth.
But listen: These always hurt us more than they hurt the other person. (The only exception might be if our anger leads to violence -- although if we're caught, we'll probably end up suffering more than they did.) In other words, bitterness is always destructive -- and the person it destroys is the person who is bitter. Your bitterness doesn't change your situation, nor does it change the person who hurt you. But it does change you -- and not in healthy ways.
This is why the Bible compares bitterness to a deadly poison, and urges us to cleanse it from our souls. If we fail to do so, we not only sin, but we pay a stiff price for our failure. No one likes to be around someone who is bitter and angry all the time. The Bible says, "Get rid of all bitterness, rage and anger, brawling and slander, along with every form of malice" (Ephesians 4:31).
How is this possible? I believe it's only possible as we submit our lives to Christ, and turn our burdens and heartaches over to Him. It may not be easy for you to do, but honestly confess your bitterness to God, and then ask Him to replace it with Christ's love and peace.