Forgiveness is the key to healing
Nothing hurts more than broken trust. When you love and care for someone and they use and abuse you and discard you as an old rag, it hurts a lot. God understands our hurt. He too was bruised and abused in relationships. We can choose to wallow in self-pity because we were hurt or we can choose to rise above it and ask God to help us love like He loves. You can allow the situation to control your life or you can take control of the situation, not with vengeance but with forgiveness.
The Bible says, "Vengeance is mine, I will repay, says the Lord." This is as it should be because only He knows the capability of people and the true motives. Only He sits as the righteous Judge in the matter because only He sees clearly the whole situation from every angle. If they are worthy of punishment for their deed, He will be just and fair in the end. But we are not the judge, He is. So we leave it in His hands.
Granting forgiveness will not come easy. In fact, in order to do it properly we need God's help. However, one thing is certain, we will not be forgiven by God until we learn to forgive. This is the clear words of Jesus in the Lord's Prayer; "Forgive us our trespasses as we forgive those who trespass against us." I need God's forgiveness, therefore I want to be more forgiving. This does not come naturally. If we want to forgive like Jesus does, we need to ask Him to help us. It is foolish to hold on to our hurts because we are the only one that is being destroyed by that action. But if we learn to let it go, God can come in and heal the wounds.
One of the keys to the release of our hurts is to pray them through. That's what a lot of the Psalms are. The writers of many of the Psalms are pouring their hearts out to the Lord. They are unloading all their ugly feelings and telling the Lord how they have been hurt and even sometimes asking the Lord to punish the offender. We need some way to release all the anger and all the hurt. Instead of unloading upon your friend, bring it to the Lord and honestly express your hurt first to Him. Sometimes He will give us insight into our own lives about our own pettiness or our own selfishness when we start expressing things to Him. Other times, just the act of bringing it to Him, "casting all our cares upon Him," will give us the comfort and relief we need.
At the end of most of those Psalms there is praise to the Lord through all the hurt. In First Samuel 30 David was blamed for a terrible mistake. Instead of bringing victory to his army, David has caused them to lose their city and all their wives and children were kidnapped. In anger and sorrow the people speak of stoning David. David was hurting and the Bible says that he "wept until there was no more power to weep." But then he encouraged Himself in the Lord. Eventually we need to turn the corner and look to the Lord.
We need to take a hold of faith. Believe that "all things work to together for good to those who love God." (Rom. 8:28) "Count it all joy when we fall into various trials knowing that the testing of your faith produces patience, but let patience have its perfect work, that you may be perfect and complete, lacking nothing." Rejoice because God will use this hurting event and turn it around for good or He will use it to grow you to help someone else who is hurting.