Happiness is Achieved by Three Things:

1) Being patient when tested
2) Being thankful when blessed
3) Being repentant upon sinning

-Imam Ibni Al Qayyim (rahimahullah)

Repentance Wipes Out Whatever (Sins) Came Before It [Islam]

Shaykh Saalih ibn Fawzaan al-Fawzaan (may Allaah preserve him) was asked the following question:

“I am a Muslim woman, al-hamdu-Lillaah. Not so long ago I did not pray and I did not know anything about matters of religion. But now, al-hamdu Lillaah, Allaah has guided me and I have started to pray, fast, read Qur’aan and recite Tasbeeh. I have completed the Qur’aan for the tenth time. Will Allaah forgive me for what I have done in the past and what I will do in the future, in secret and openly in my life? What more can I do so that Allaah will forgive me?” 

The response was: 
Repentance wipes out whatever (sins) came before it. So long as you have repented sincerely – al-hamdu Lillaah – and you are doing the duties enjoined upon you by Allaah and avoiding what He has forbidden, then Allaah will forgive whatever came before that repentance. Allaah says (interpretation of the meaning):
“Say: “O ‘Ibaadi (My slaves) who have transgressed against themselves (by committing evil deeds and sins)! Despair not of the Mercy of Allaah, verily, Allaah forgives all sins” [al-Zumar 39:53] – even shirk, if the person who committed shirk then repents from it to Allaah, He will accept his repentance. Allaah says (interpretation of the meaning):

“Say to those who have disbelieved, if they cease (from disbelief), their past will be forgiven” [al-Anfaal 8:38]. 

The Prophet (peace and blessings of Allaah be upon him) said: “Islam wipes away whatever (sins) came before it, and repentance wipes away whatever (sins) came before it.” (Narrated by Ahmad, 4/204). If you have repented sincerely and you do what Allaah has enjoined upon you and avoid that which He has forbidden, this will be sufficient in’sha’Allaah for your past sins to be forgiven, but you must do good deeds in the future, and keep on repenting and obeying the Islamic commandments which Allaah has enjoined upon you. 

It is to be noted that the person who deliberately does not pray is a kaafir, if he does not pray at all; but if he repents and prays, he comes back to Islam, and he does not have to make up what he has missed, but he is advised to do a lot of naafil prayers.

And Allaah knows best