I was engaged to someone. Later on he told me that he wanted to marry me just because I was a nice practicing Muslim girl. He lied to me, cheated on me, broke my heart and left me alone. I feel so cheated and betrayed. His betrayal is killing me every second. I feel so deceived. I am in so much pain. I am praying so much to Allah so that He will set me free from this pain and sorrow. Is there any special dua to remove pain and sorrow which would harden my heart? I feel so deceived. I can't take this suffering anymore.
I am really very sorry to hear your plight. It is not expected from a man of good moral character to cheat others no mention of such near relations. You have sought right path. Continue praying to the Almighty to grant you ease and make this man amend for his past behavior and treat you fairly. God can do anything and we must not lose hope and should keep trying to change the circumstances and win the hearts of the related people. Being depressed would not help you much.
I am afraid I cannot suggest any set words for the purpose. It is not compulsory for one to offer supplication in some set and recognized wording. Not any such popular Dua guarantees acceptance from the Almighty. However, if we find words fully express our needs before God from the Qur'an or the Prophetic sayings that is best for us but that does not necessarily mean that our supplications are only accepted this way. We prefer them for the words used by the Almighty in the Holy Qur'an and the ones said by the Holy Prophet best convey our need with express humility yet it does not form criterion for its acceptance. They are not magical spells, which can make God act as uttered by us.
It is only the absolute trust in the Almighty and humble supplication with all sincerity that is certain to convey our requests to the Almighty. I would suggest that whoever is in need could present his request to the Almighty in any language he knows. Even the language is not the sole mode of communication to the Almighty for he could read your thought before they are formed. It is only our need that we have to present our thoughts in words and utter them to develop a feeling that we have communicated something to another person.
May Allah bless you with all happiness and remove your pain and grant you success in both the worlds!