Conditions For Entering Heaven or Hell at the Day of Judgement

As Salaam alikum,
Hasten in repentance, do it right now, because you never know when death is going to take over you.
And it is for sure that your sins are not bigger than the Mercy of Allah (s.w.t), so do not despair of HIS mercy.
http://islamqa.com/en/search/repentance/AllWords/t,q,a

Assalaam Alikum

Brother, do not ever feel this way, remember Allah is Most Merciful and Oft-Forgiving.

When you pray, you are being forgiven of your sins. When you go through hardships in life, you are being forgiven of sins. You are constantly being forgiven of them without even knowing.

Repent for any specific sins that you can remember and also do not forget to
1. acknowledge what you did was wrong
2. ask for forgiveness
3. make an earnest attempt not to let it happen again

For sins you can't remember, just in general ask Allah to forgive you of your sins. Say "Astighfirallah" often.

Regarding "clean slates" I believe there are Durood Sharifs you can recite which clear 80 years of sins. Unfortunately I do not know how authentic the claim is, perhaps someone can fill us in on this.

But there is an authentic hadith for something like this. But remember, do not say them and continue indulging in sins, they are only good if you try harder to better yourself.
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Durood-e-Wali

ALLAHUMMA SALLI ALA MUHAMMADININ NABI YIL UMMI YI WA AALIHI WABAARIK WA SALLIM.

"O Allah! Send blessings on Muhammad, our chief the unlettered Apostle and his Family Thy favours and thy Salutations".

Hazrat Shah Wali'ullah Dehlvi (radi Allahu anhu) said that his father commanded him to recite the above Durood Shareef as it was the best one.

Hazrat Abu Hurairah (radi Allahu anhu), a close companion of Sayyiduna Rasoolullah (sallal laahu alaihi wasallam), has said that if a person recites this Durood eighty times on Friday (after evening prayers), then his sins equivalent to 80 years will be forgiven.

This is a very important Durood Shareef which brings forth immediate success and satisfaction to the heart and the mind.

Hazrat Zarar bin Auzar (radi Allahu anhu), another companion of the Holy Prophet (sallal laahu alaihi wasallam) says that he used to recite this Durood Shareef and fight with the non-believers. In every battle, he saw that success kissed his feet, and he never lost any battle on any day.

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Sahih Bukhari Volume 8 : Book 75 : Hadith 414 :

Narrated by Abu Huraira:

Allah's Apostle said, "Whoever says, 'Subhan Allah wa bihamdihi,' one hundred times a day, will be forgiven all his sins even if they were as much as the foam of the sea.

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I hope this helps brother, but please remember, one of the most important aspect of Allah is that He is very forgiving. So please when you ask for forgiveness, leave thinking Allah has forgiven and do not let Shaytaan doubt this.

The sins are recorded starting from puberty or the time where one can properly differentiate right from wrong and this goes on until you die that is correct. So someone who is mentally ill and does not know right from wrong is not held responsible for such actions.

There is no starting over in the new year or date really, that I know of. But brother when you do something bad it counts as 1 sin. When you want to do something good and do not do it, is counts as 1 good deed. When you want to do something good and you actually DO it, it can count as 10 good deeds. And the hadith I referred to you, basically, you can inshAllah be forgiven of all your sins (throughout your life) everyday and of every hour if you wanted to! Do not feel hopeless, Allah is most merciful, honestly, it makes me cry sometimes.

And Allah knows best.