Ask for forgiveness immediately

قَالَ رَبِّ إِنِّى ظَلَمْتُ نَفْسِى فَٱغْفِرْ لِى فَغَفَرَ لَهُۥٓ ۚ إِنَّهُۥ هُوَ ٱلْغَفُورُ ٱلرَّحِيمُ

He said: "My Lord! Verily, I have wronged myself, so forgive me." Then He forgave him. Verily, He is the Oft-Forgiving, the Most Merciful. http://www.muslimpro.com 28:16 Qasas

So Musa kills a man and asks for forgiveness immediately and gets it. Proof that Allah is ever with the believers and a thorough explanation of how to deal with sin.

1. Make tauba Immediately.
2. Do not return to the action and change your behavior/attitude.
3. Realize that Shaitan is going to get in your face to invoke despair. He will cause you to fear the creation (the economy, the law, your job, a person,etc...) more than Allah.
4. Remove yourself from the situation.
5. Submit completely to the help and mercy of Allah, and pray.

Use this formula and succeed in this world and the next.